Enable hyphen separated range of atoms to modify mass

[marcelmbn]

Refers to the following feature

Situation before

before, only single entries like:

$hess
modify mass: 12, 2.0, 3, 4.0
scale mass: 2, 1.5, 5, 6.0
$end

were possible.

Improvement

Hyphen-separated ranges of atoms can be read in, like:

$hess
scale mass: 12-50,2.0
modify mass: 1-3,400.0
$end

[marcelmbn]

Renamed ati to iat to prevent confusion with atom type. (Variable represents the numbered atom in the range from iat = 1,nat, and not the type of atom, i.e. the proton number)