read all the way through concatenated pdb files from ChimeraX

[jamesmkrieger]

If two pdb files are loaded into ChimeraX, they can be joined into one file there, but it's not really a proper PDB file and ProDy doesn't read it properly as a consequence.

It has header and atom section from file 1 and then header again for file 2 before atoms from file 2. It would be nice if ProDy could read these somehow.

We may want options about whether to do this or not and whether to add the corresponding atomgroups together or not.