Add support to specify key type and always decode level as float

[alexamici]

With this change you can optionally specify the key type when using Message.__getitem__ like:

>>> msg["level:float"]
1.0
>>> msg["level:int"]
1
>>> msg["level"]
1

Then the level key is decoded as float in order to close #195 . Note that this is a significant user-visible change but I think it is the best default.

[iainrussell]

Sorry @alexamici I missed this PR. It looks good to me, but it would look even better if there were some tests, if I may be so cheeky :)

[alexamici]

I'm now hesitant to add this feature to cfgrib as it really looks like a lower level functionality. @shahramn do you have a way to specify the target data type in the key, like msg["level:float"]. Would you be interested in addign this functionality to eccodes-python?

[shahramn]

I'm now hesitant to add this feature to cfgrib as it really looks like a lower level functionality. @shahramn do you have a way to specify the target data type in the key, like msg["level:float"]. Would you be interested in addign this functionality to eccodes-python?

We already have a way of passing in the type information. For example:

% python3
>>> import eccodes
>>> f = open(input, 'rb')
>>> gid = eccodes.codes_grib_new_from_file(f)
>>> print( eccodes.codes_get(gid, 'level') )
65
>>> print( eccodes.codes_get(gid, 'level', float) )
65.4
>>> print( eccodes.codes_get_double(gid, 'level') )
65.4

[codecov-io]

Codecov Report

Merging #210 (c253090) into stable/0.9.9.x (919a513) will decrease coverage by 0.08%.
The diff coverage is 80.00%.

@@                Coverage Diff                 @@
##           stable/0.9.9.x     #210      +/-   ##
==================================================
- Coverage           92.29%   92.20%   -0.09%     
==================================================
  Files                  25       25              
  Lines                1714     1720       +6     
  Branches              201      202       +1     
==================================================
+ Hits                 1582     1586       +4     
- Misses                110      111       +1     
- Partials               22       23       +1     

Impacted Files	Coverage Δ
cfgrib/messages.py	93.16% <66.66%> (-0.61%)	⬇️
cfgrib/dataset.py	98.93% <100.00%> (+<0.01%)	⬆️

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[alexamici]

@iainrussell I added the tests for the generic functionality. This is ready to merge for me.

[iainrussell]

Looks good to me, great that eccodes has such a nice interface to allow this to work so conveniently!