add pdb download functionality

[alisterburt]

@mgiammar also removed unused structure_name from mmdf.write()

[codecov]

Codecov Report

❌ Patch coverage is 81.96721% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.39%. Comparing base (81bf8d8) to head (e9028b9).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mmdf/_pdb_utils.py	79.24%	11 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (81bf8d8) and HEAD (e9028b9). Click for more details.

HEAD has 61 uploads less than BASE

Flag	BASE (81bf8d8)	HEAD (e9028b9)
	65	4

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   96.22%   88.39%   -7.84%     
==========================================
  Files           3        4       +1     
  Lines          53      112      +59     
==========================================
+ Hits           51       99      +48     
- Misses          2       13      +11     

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[mgiammar]

Nice functionality! I've serendipitously also discovered the PDB download API but have been wget-ing my structures.

[alisterburt]

hehe - will be nice! this also caches the downloads, I have some simulation workflows where this should make providing examples easier, imagine it might be nice for 2DTM too!