Column names for mutation counts and frequencies are inconsistent

[Jason Vander Heiden]

observedMutations outputs column names of the form:

• Counts: OBSERVED_<Region>_<R/S>.
• Frequencies: MU_FREQ_<Region>_<R/S>

I know this is because of BASELINe, but it's weird. Counts should be something like MU_COUNT_<Region>_<R/S>. Similarly, we should probably rename the expected mutation calls to something like MU_EXPECTED_<Region>_<R/S>

[Julian Zhou]

OBSERVED -> MU_COUNT

EXPECTED -> MU_EXPECTED

via commit 5f5ed06.