Fixes the parsing of PBD coordinate files

Description

The parsing was wrongly searching for 'HETATOM' instead of 'HETATM'.

The unit is also now always assumed to be in Angstrom, as specified in the standard.

This is related to #653 (closed). There is still a test missing for these two variables, so this issue should not be closed.

News snippet

Fixes the parsing of PBD coordinate files

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.