Rename some fields read from PDB, to be more in line with PDB conventions

[tovrstra]

For example, use atname instead of attype. Normally, the field from which it is read is an atom name, not an atom type. Same issue for resname versus restype.

See PDB spec: https://www.wwpdb.org/documentation/file-format-content/format33/sect9.html#HETATM

Originally posted by @tovrstra in #284 (comment)

[Aditish51]

Hey @tovrstra Can I work on this? It seems that just renaming has to be done which I think I can do.

[PaulWAyers]

I think that @evohringer may also be able to help guide this effort.

[tovrstra]

Sounds good. I'm happy to review.