Add an AOT retrieval as an alternative to the 3-phases of water retrieval in the AC over land.

Currently, the ground segment provides a mean AOT value per EnMAP tile. This could be replaced by an AOT retrieval within SICOR. However, it must be implemented as an alternative to the 3-phases-of-water retrieval there.

Steps to do:

• wait for a respective implementation in SICOR
• add a config parameter to choose between AOT retrieval and the 3-phases-of-water approach