ppImageSubtractBackground misbehaving

[eugene]

ppImage seems to be doing some odd things for the masked pixels for reasons I don't understand:

• ppImageSubtractBackground does not seem to respect the DETECTOR bit value (areas masked with only this bit are not modified correctly)
• ppImageSubtractBackground does not quite do the right thing with other areas: some masked values seem to be set to nan, but other seem to be set to strange values, eg, modestly negative values
• ppImageSubtractBackground needs to be able to optionally replace the background in the masked areas, as it used to do (when it was named ppImageReplaceBackground). sometimes we want one option, sometimes the other (for example, to make pretty images, we need to keep the background and have the masked values go to a reasonable value -- an interpolation would probably be even better).

[Paul Price]

I think Gene has fixed this. There was a discrepancy between what ppImage was doing, and what psphot does.