Added io.reader.Solar_Irradiance_reader().
Added model.srf.py for handling spectral response functions + test_srf.py.
Extended metadata reader with srf and solar_irrad attributes.
Refactored TOARad2TOARef_Transformer to Radiometric_Transformer.
Removed deprecated dummy TOARef transformer.

Updated __version__ and __versionalias__.

contains:
- "examples" subpackage: to be used for all software examples, e.g. IPython notebooks demonstrating how code is to be used
- within "enpt" package:
    - "execution" module: all code related to execution of function, e.g. the process controller
    - "model" module: our dataset representation, e.g. for EnMAP images with different processing levels
    - "processors" module: submodules with all processors needed within EnPT
    - "utils" module: to be used for geoprocessing tools, path generators, loggers, ...
- "tests": added test submodule for radiometric_transform processor