Add skip logic if products already exist and generate only WTR water gain maps

[cmspeed]

This PR adds simple logic to check if a mosaic, difference map, or flood extent map exists in the directory prior to generating it. This is helpful if the processing was interrupted previously, as it allows the processing to resume where it left off. Also added here is a step to ensure water_gain maps are only generated for WTR layers.

Changes

• Added path.exists() checks in pipeline.py and layout.py to determine if products by that name already exist. If so, skip product generation.
• Added logic to ensure water_gain maps are only generated from the WTR layer (and not BWTR).

[ehavazli]

adding option to overwrite (or not) would be better. Implemented behavior in this PR can cause confusion and doesn't offer a solution to rewrite files.

[cmspeed]

adding option to overwrite (or not) would be better. Implemented behavior in this PR can cause confusion and doesn't offer a solution to rewrite files.

Thanks for pointing this out @ehavazli. A new flag, --skip-exisiting has been added to let the user decide if they would rather skip or overwrite existing files.

[ehavazli]

looks good