This change adds support for newer functions that are prefixed
by _xlfn. (#356). The calculation engine has been updated to
recognise these as functions, and drop the _xlfn. part.
It also add a couple of the new functions such as STDEV.S/P,
MODE.SNGL, ISFORMULA.
Fixes#356Closes#390
Because even if it doesn't make a difference in practice, it is
technically more correct to call static methods statically. It
also better advertise that those methods can be used from any context.