Clarify variables for right ascension and declination.

[cnsetzer]

At present it is not entirely clear that _ra and _dec in the summary tables should be the variables used which should be agnostic of OpSim version. This could be clarified so that users do not accidentally use fieldRA/ditheredRA assuming these are the correct columns which undergo unit conversion etc.

[rbiswas4]

@cnsetzer As we discussed, it is not usually a good idea to have underscored variables fo users. Indeed, when developing this, these variables were meant to be internal to OpSimSummary and not used by the user at all. A user was expected to use the syntax described here to obtain cadences.

Since you find this useful, it might be good to define public variables related to _ra and _dec but following the broader LSST convention, must be in degrees . Do you think this will serve your purpose? I can then add these (even though there is some extra memory usage in pandas).