Add argument checks, allow Nothing type for optional args

[vlandau]

• Add check for reclassify_resistance = false when reclass table is provided. Raise warning if needed. reclass_table should give a warning if reclassify_resistance is false #145
• Check that conditional is set to true if user provides paths to conditional rasters, throw error otherwise
• Check that user provided a path to a source file if source_from_resistance is false, throw an error if not Julia/Omniscape not recognizing files [Solved: ini file mistake - no source specified] #147
• Raise warning if project_name already exists, and print location of outputs, and update notes in docs. project_name is not respected when doing successive runs #146
• If any conditional rasters/paths were provided but won't be used due to misspecified config options, throw an error

Check that only appropriate values are set for:

• compare_to_future
• comparison1
• comparison2