Added a file physics.c with routines that convert values from and to

Added a file physics.c with routines that convert values from and to
gromacs units. The physics.h header file describes which units are supported
(in a list of enums).
Example:
x = convert2gmx(y,eg2cAngstrom);
will convert y in Angstrom to gromacs units (nm). Back transform by
y = gmx2convert(x,eg2cAngstrom);