Fix isotope hessian

[marcelmbn]

Fixes issue #1143

Problem

• Isotope masses were overridden by physical values.

Fix

• shift initialization of atomic masses directly to line directly after allocation of the array atmass (see init_split).

[marcelmbn]

General question, @Albkat @awvwgk: We don't have any validation tests that access the CLI of xtb, i.e. tests that actually execute the xtb binary with a call like the following, right?

xtb --hess struc.xyz

In this way, we could also test this feature. Up to now, this can easily be broken again by some PR that works on main.F90 and doesn't know about such features and doesn't check them manually.

[marcelmbn]

@demonic-daisy @MtoLStoN Maybe you can look quickly into this to check if the sandwich potential thing could be affected by this (but I don't think so).