Allow atom types to be included in PDB reading

**Is your feature request related to a problem? Please describe.**
MDMC can currently only read in cif files with the option of passing in atom types to the read-in configuration. This is not yet implemented for PDB configuration readers.

**Describe the solution you'd like**
Implement such functionality into the reading of PDB files, where the user can enter the atom types to be applied to the final configuration in an easy way, and this is then applied to the final configuration

**Describe alternatives you've considered**
Perhaps use some metadata/comments in the PDB file(s) to convey the same information, but this does may not exactly comply with the PDB standard, making reading/exporting more difficult.