uvotpy: UVOTY-2.0.2 updated coincidence loss consistent with theory

The Swift UVOT grisms (uv: 170-500nm; visible: 285-660 nm) spectral reduction package is a replacement for the uvotimgrism Ftool from the HEADAS Swift software. Requires a recent HEADAS Swift installation. Recently the coincidence loss correction was redeveloped and is now formulated in a fully consistent manner to the theory as used successfully for point sources. Updates to the calibration files were made consistent with the reformulated correction. The background algorithm was extended to be more accurate for some difficult images. A program was added to help shift the wavelengths due to uncertainties in anchor position.