`load_pdb`: support alternate locations other than `A`

[tkemmer]

Structures such as 1HX0 contain fragments that only appear at an alternate location (other than A):

HETATM 4180  C1 AGLC B   1      43.566  24.157  57.826  0.76 28.81           C  
HETATM 4181  C2 AGLC B   1      43.954  22.844  57.134  0.76 30.75           C  
...
HETATM 4190  O5 AGLC B   1      42.375  23.960  58.622  0.76 30.32           O  
HETATM 4191  O6 AGLC B   1      40.260  22.784  59.866  0.76 43.68           O  
HETATM 4192  C2 BBGC B   1      43.954  22.844  57.134  0.24 30.75           C  
HETATM 4193  C3 BBGC B   1      42.748  22.328  56.383  0.24 24.17           C  
...
HETATM 4202  O5 BBGC B   1      42.375  23.960  58.622  0.24 30.32           O  
HETATM 4203  O6 BBGC B   1      40.071  24.536  59.746  0.24 36.59           O  

Our current PDB reader ignores fragment BGC at location B (as well as all of its atoms) as there is no version of it at location A.

[tkemmer]

This affects almost 100k PDB IDs