Create regression test for FaparLimitation class

[MarionBWeinzierl]

In #403 , we implemented the FaparLimitation class to calculate LAI fAPAR_max values. We added some unit tests and an example code, but not yet integration/regression tests. This is issue is for implementing those regressions tests. We are doing this by

• writing out the required annual, already "wrangled" data from the example from Annual fAPAR_max and LAI_max #403 into a netcdf file
• writing a method to read in the data
• writing a regression test using the class constructor
• writing out the data for the from_pmodel class function
• writing a method to read in the data for from_pmodel
• writing a regression test using the from_pmodel class function