supports specifying disconnected omega ranges in config file

This pull request allows disconnected omega ranges to be specified in the config file.

@joelvbernier, is this sufficient for you to modify the analysis routines to support disconnected ranges? Ideally, one would just use `cfg.image_series.omega.positions`, but `start`, `stop`, `step`, and `steps` are also available if they are needed. The latter properties currently may return either lists or individual values, depending on how they are defined in the yml file. This could be tweaked so the config class always returns lists, it might simplify the API a little.