Create DolphotTable.export_for_starfish method

This method can be used to create StarFISH photometry input files.
These are relatively bare-bones files, with two columns giving
photometric values along the x and y axes of the CMD or CCD plane.
Select stars in StarFISH CMD plane for export

StarFISH wants CMD catalogs to only have stars that are within
the x and y spans of the CMD plane.