Add warnings and improve documentation

Warnings should be added for the following cases:

• small input image dimensions
• matching window size set to very small value (low accuracy) - !70 (merged)
• geocoding information of input images is not provided to COREG or COREG_LOCAL
• too many points were filtered out due to exceeded shift distance
• grid size is very small (risk of spatial artifacts in the output)

Documentation should be improved regarding:

• a meaningful input image dimension
• a meaningful matching window size
• parameters of COREG and COREG_LOCAL (add more prominent link to API docs)
• optimal grid size (drawbacks of too dense grids)
• how to optimally set filtering thresholds with respect to visualizations
• how to judge if tie points are equally spread
• how to handle different acquisition angles of input images