Feature puma patches [2/x] add element ref coords interface

[sandro-elsweijer]

Describe your changes here:
The interface for the coming element ref coord functions. The function gets a set of reference coordinates inside an element $[0, 1]^d$ and maps these coordinates to the reference coordinates of the tree $[0, 1]^d$, regarding the anchor coordinate, level and orientation. This way, we can evaluate the geometry of arbitrary points inside an element. Before, we could only evaluate the vertices of the elements.

All these boxes must be checked by the reviewers before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

• The author added a BSD statement to doc/ (or already has one)

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
  • If not, does the specific use case compile and all tests pass (check manually)

• All tests pass (in various configurations, this should be executed automatically in a github action)

• New source/header files are properly added to the Makefiles

• New Datatypes are added to t8indent_custom_datatypes.txt

• The reviewer executed the new code features at least once and checked the results manually

• The code is covered in an existing or new test case

• New tests use the Google Test framework

• The code follows the t8code coding guidelines

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue)

• If a new directory with source-files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.

• If this PR introduces a new feature, it must be covered in an example/tutorial and a Wiki article.

I still have to think about it.