modularization of SAR processing engine

[johntruckenbrodt]

Currently SNAP is tightly integrated into the processor. This makes it hard to add alternatives. A better modularization could look like this:

• add a dedicated section SNAP to the config.ini file gathering
  • relevant configuration from config.snap_conf (move the rest somewhere else)
  • the configuration parameter snap_gpt_args
• remove the option to define multiple processing sections in the config.ini. This makes the file too complicated. One section PROCESSING, one METADATA and one per processing engine is enough (solved in start processing with 's1rb process'; new option 's1rb init' to initialize config file #215)
• add a configuration parameter, e.g. engine, to let users define the engine. Its name should match a file <name>.py and a section in config.ini should have the same name in uppercase

The config.ini file could look like this:

engine = 'snap'

[SNAP]
gpt_args = 
...

The script processor.py could look like this:

import importlib
engine = config['engine']
processor = importlib.import_module(engine)
...
processor.process(..., **config[engine])