Better names for `mt2mhz` and `mhz2mt`

[stestoll]

The names of these two functions are based on units rather than the associated quantities and therefore not ideally named.

field2freq and freq2field would be better.

Also, there is a bit of overlap (or a chance for a merger?) between field2freq and larmorfrq.

eprconvert is related.

[stestoll]

One idea is to introduce a more general unit conversion function unitconvert:

unitconvert(1e-4,'cm^-1->K')
unitconvert(1e-4,'cm^-1->MHz')
unitconvert(350,'mT->MHz')
unitconvert(350,'mT->MHz',2.1)
unitconvert(350,'mT->MHz','1H')
unitconvert(3500,'G->mT') 
unitconvert(350,'mT->MHz') 

mt2mhz(350)
unitconvert(350,'mT->MHz')

mt2mhz(350,2.1)
unitconvert(350,'mT->MHz',2.1)

The unit string should probably be interpreted in a case-insensitive manner, i.e. mT->MHz and mt->mhz should both be understood.

Note that Matlab has a unitconv function as well as a unitConvert function (Symbolic Math toolbox).

larmorfrq should be kept separate, since it's such a fundamental notion in magnetic resonance.

[stestoll]

Implemented by https://github.com/StollLab/EasySpin/pull/294