Add ability to create columns in Tracer that are expressions of other columns

[rambaut]

From @msuchard on August 13, 2015 17:46

Originally reported on Google Code with ID 210

Put 'add calculated column' into Tracer. 

Reported by rambaut on 2009-10-15 18:19:57

Copied from original issue: beast-dev/beast-mcmc#219