Equatorial coordinate systems for 1950.0 and 2050.0 epochs

[alex-w]

Original report: https://bugs.launchpad.net/stellarium/+bug/1361678

Original feature request and discussion: https://sourceforge.net/p/stellarium/discussion/278769/thread/5e4f6a05/

It would be great to have these implemented. The 1950.0 epoch coordinates prove especially useful, as legacy sources (most from the early 1980s and older) used those rather than 2000.0. As for 2050.0, it will supersede 2000.0 within the next few decades, so it certainly wouldn't hurt to anticipate this and implement it as well.