Add change mass/charge command.

[drroe]

Version 6.20.4.

Add ability to change mass/charge to the change command:

change {mass|charge} [of <mask>] {to <value>|fromset <data set>}

So to e.g. change the masses of atoms to the results of an atomicfluct calculation:

parm ../AFV.parm7
trajin ../AFV.nc
align first !@H=
atomicfluct MyFluct #out AFV.myfluct.dat
run
change mass fromset MyFluct

Adds tests and update manual.