We should change the way dithers are added in `OpSimOutput` and `make_simlibs`

[rbiswas4]

Currently, the way opsim summary desires to add dithers are:

• use dithers in opsim table in column ditheredRA etc.
• use an externally provided set of dithers
• generate dithers internally using a prescription of random numbers.

The way these options are used is convoluted and should be cleaned up. (See comments at opsim_out.pyaround line 548.