Added feature that generates the elementary weight of a rooted tree as a latexstring

[pw0lf]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Added the feature as described in #96.
Also added test and they all passed.
Added documentation, but the picture of the output is still missing, since I don't remember how to add pictures there ...
This picture should be displayed:

[pw0lf]

Almost forgot:
Because a function called elementary_weight already existed, I called the function elementary_weight_latexstring.
Should I rename elementary_differential to elementary_differential_latexstring out of consistency?

[ranocha]

Almost forgot: Because a function called elementary_weight already existed, I called the function elementary_weight_latexstring. Should I rename elementary_differential to elementary_differential_latexstring out of consistency?

Good point 👍 Could you please rename the function as suggested - and deprecate the old one?

help?> @deprecate
  @deprecate old new [export_old=true]

  Deprecate method old and specify the replacement call new, defining a new method old with the
  specified signature in the process.

  To prevent old from being exported, set export_old to false.

  │ Julia 1.5
  │
  │  As of Julia 1.5, functions defined by @deprecate do not print warning when julia is
  │  run without the --depwarn=yes flag set, as the default value of --depwarn option is
  │  no. The warnings are printed from tests run by Pkg.test().

  Examples
  ≡≡≡≡≡≡≡≡

  julia> @deprecate old(x) new(x)
  old (generic function with 1 method)

[pw0lf]

I changed the name and deprecated the old function. Also changed the version number to 3.0.0 since it is a breaking change, right?

Also the picture should be added to the documentation and I don't know how...
Could you help me with that, @ranocha?

[ranocha]

Also the picture should be added to the documentation and I don't know how...

You can insert some code similar to

RootedTrees.jl/docs/src/tutorials/basics.md

Lines 83 to 85 in e9698cf

	In LaTeX this results in the following output:
	

In this case, the link to your picture above is https://private-user-images.githubusercontent.com/125130707/298310491-8a035faf-fd1a-4fc0-92be-c3387eb53177.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJnaXRodWIuY29tIiwiYXVkIjoicmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbSIsImtleSI6ImtleTUiLCJleHAiOjE3MDYxNjY1ODAsIm5iZiI6MTcwNjE2NjI4MCwicGF0aCI6Ii8xMjUxMzA3MDcvMjk4MzEwNDkxLThhMDM1ZmFmLWZkMWEtNGZjMC05MmJlLWMzMzg3ZWI1MzE3Ny5wbmc_WC1BbXotQWxnb3JpdGhtPUFXUzQtSE1BQy1TSEEyNTYmWC1BbXotQ3JlZGVudGlhbD1BS0lBVkNPRFlMU0E1M1BRSzRaQSUyRjIwMjQwMTI1JTJGdXMtZWFzdC0xJTJGczMlMkZhd3M0X3JlcXVlc3QmWC1BbXotRGF0ZT0yMDI0MDEyNVQwNzA0NDBaJlgtQW16LUV4cGlyZXM9MzAwJlgtQW16LVNpZ25hdHVyZT1kYzQyYjM0MWY0MDU5YWZlYzBmODA4MjFiZGIxN2E3YjhkYTdmZDNkYTU5NmI5OTEwNWFiZjg0OGZjNDg1MzZhJlgtQW16LVNpZ25lZEhlYWRlcnM9aG9zdCZhY3Rvcl9pZD0wJmtleV9pZD0wJnJlcG9faWQ9MCJ9.GB-PigOlQqenzgruzWg19qslzM6RXeX4xWwCNreOvNY

[ranocha]

The code is something like

![latex elemenary weights](https://private-user-images.githubusercontent.com/125130707/298310491-8a035faf-fd1a-4fc0-92be-c3387eb53177.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJnaXRodWIuY29tIiwiYXVkIjoicmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbSIsImtleSI6ImtleTUiLCJleHAiOjE3MDYxNjY1ODAsIm5iZiI6MTcwNjE2NjI4MCwicGF0aCI6Ii8xMjUxMzA3MDcvMjk4MzEwNDkxLThhMDM1ZmFmLWZkMWEtNGZjMC05MmJlLWMzMzg3ZWI1MzE3Ny5wbmc_WC1BbXotQWxnb3JpdGhtPUFXUzQtSE1BQy1TSEEyNTYmWC1BbXotQ3JlZGVudGlhbD1BS0lBVkNPRFlMU0E1M1BRSzRaQSUyRjIwMjQwMTI1JTJGdXMtZWFzdC0xJTJGczMlMkZhd3M0X3JlcXVlc3QmWC1BbXotRGF0ZT0yMDI0MDEyNVQwNzA0NDBaJlgtQW16LUV4cGlyZXM9MzAwJlgtQW16LVNpZ25hdHVyZT1kYzQyYjM0MWY0MDU5YWZlYzBmODA4MjFiZGIxN2E3YjhkYTdmZDNkYTU5NmI5OTEwNWFiZjg0OGZjNDg1MzZhJlgtQW16LVNpZ25lZEhlYWRlcnM9aG9zdCZhY3Rvcl9pZD0wJmtleV9pZD0wJnJlcG9faWQ9MCJ9.GB-PigOlQqenzgruzWg19qslzM6RXeX4xWwCNreOvNY)

[ranocha]

Thanks a lot! It's weird that docs CI fails. I will check it in another clean PR (#157)