Feature/separate ac land water

This adds a separate atmospheric correction for water surfaces based on POLYMER.

Tasks to be solved before we can merge:

• fix test_styles CI job (run make lint in the root dir of EnPT locally and check the logs)
• implement acwater as optional requirement, i.e., auto-disable it if acwater + POLYMER is not installed
• redirect water EnMAP pixels to acwater and land pixels to SICOR IF acwater + POLYMER is installed
• combine atmospherically corrected results after running acwater and SICOR
• evaluate range of output in combined mode
• add all needed configuration parameter for acwater to config.py, options_default.json, options_schema.py
• update the EnPT console argument parser (enpt_cli.py) with the now parameters needed for acwater
• add documentation regarding acwater in installation.rst, algorithm_descriptions.rst, about.rst, README.rst and maybe also something in tutorial.rst
• add @brenner.silva_at_awi.de in AUTHORS.rst as soon as ICLA/CCLA is signed
• resolve all tasks and discussions in #47 (closed)
• include acwater and POLYMER in the CI runner to make automated software tests work
• provide software tests that cover acwater
• validate EnPT L2A results over water areas

Closes #47 (closed).

@brenner.silva_at_awi.de @nbohn:
Lets develop this further in this merge request. Simply continue pushing to feature/separate_ac_land_water and check the above boxes as soon as these tasks are done.