Save disk space by cleaning up decompressed files independently from legacy script succeeding or not

[maindlt]

Save disk space by cleaning up decompressed files independently from legacy Fortran code succeeding or crashing.

• Moved deleting the decompressed files from the shell script to the method LegacyWeightSolver.solve() so deletion takes place independently of any errors that might occur in the shell script and/or the legacy executables called by that script.

Safer removal of nn_orbmat.out: check for existing file before deleting.

Suggestion for testing:

1. Provoke an error in LegacyWeightSolver.solve(), right after executing the shell script (this simulates a script error). For example, this can be done in weight_solvers.py by changing line 316 from if not p.stdout.decode("UTF-8"): to if not p.stdout.decode("UTF-8") and False:.
2. Run any example using the LegacyWeightSolver.
3. Verify that after DYNAMITE crashes, there are no surviving orblib_*.dat files in any of the datfil/ folders (those would be the uncompressed files).
4. Optional: in line 314, change os.remove(f_name) to pass and re-run. The orblib_*.dat files should still be there in the datfil/ folder(s) after DYNAMITE crashes.

So far, tested successfully with test_nnls.py (after changing to the LegacyWeightSolver in its config file).

[prashjet]

I ran the suggested test with test_nnls and all orblib_*.dat files are removed.

[maindlt]

Many thanks, Prash! Will merge and delete branch :-)