Fix situation with pruned trees and Random Local Clocks

[lmaurits]

When using PrunedTrees/PrunedAlignments for datasets with lots of missing data, non-strict clock models need special treatment to ensure that everything still works correctly. Joseph has written a PrunedRelaxedClock class, and BEASTling uses this when appropriate. However, no PrunedRandomLocalClock class exists yet. We should (i) first change BEASTling to switch off PrunedTrees for analyses with RLCs (and inform the user), and (ii) sometime in the nearish future write the PrunedRandomLocalClock class and get it in BEASTLabs.

[lmaurits]

I have implemented (i) above, i.e. pruned clocks are disabled for RLC analyses and the user is informed of this if using --verbose. (ii) is still something I would like to do, but it's not an urgent goal. I'm not going to close the issue, so that there is a reminder to do (ii) someday, but I am going to remove the 1.2.0 milestone because I think (i) alone is good enough to solve this for 1.2.0, as it will prevent people running flawed analyses.