Refactor code such that PfsConfigs are not dark/flat/science specific

Following SIM2D-94 it was noticed that there is code in makePfsConfig.py to make flat or dark-specific PfsConfig s.

As cloomis had remarked in PR #3 for the above issue:

I do not think that pfsConfig can or should hold any information about what light goes through a fiber by declaring makeFlatConfig, makeDarkConfig, etc. The design tells where the cobras are pointed (which can be behind a spot), but that same config is used for taking darks, arcs, flats, and science exposures.

Please make the generated pfsConfigs independent of flats, darks or science.