Harvester parsers as plugins

[mjaquiery]

The harvesters' parsing scripts are included directly within the harvester code.
Harvesters would be more extensible if they could rely on Python library plugins as parsers.

• Register parser plugins somewhere
• Dynamically download missing plugins
• Provide a mechanism for determining which plugin to use on which files
• Release code for current parsers as separate python packages
• Solidify and plublicise InputFile class as generic target class for plugins
• Provide documentation on how to write custom plugins
  • Provide description of InputFile data representation to support conversion to different output formats

Theoretically, these plugins could be enabled/disabled at runtime from Galv.