Recognize that parse_calibration_string is a generic prior parser
Recognize that parse_calibration_string is a generic prior parser

Also, it is actually a static method, which gives us the opportunity
to move it somewhere else, so we can have all the prior parse and XML
logic in one place, instead of somewhere in configuration.py and
somewhere in beastxml.py and to a small extent in beast_maps.py as it
is now.

lmaurits/BEASTling