Merge beast.evomodel and beast.beagle

[armanbilge]

It seems very redundant to have separate evomodel interfaces/implementations for Beagle.

In fact, I see that the Beagle Jar has its own pure-Java implementation. This means that we should be able to remove the BEAST implementations and rely entirely on Beagle, regardless of whether it is actually installed on the system.

[armanbilge]

After speaking with @alexeid today I learned that the highly-optimized Beagle implementation may return values that are inconsistent with various reference implementations (such as the BEAST implementations). Therefore it is valuable to keep the BEAST implementations, although I hope it is still possible to merge these packages at least to some extent.