Add new open method

[banesullivan]

import localtileserver as lts
client = lts.open('file.tiff')

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.04%. Comparing base (df43e54) to head (b1c638a).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
+ Coverage   83.93%   84.04%   +0.10%     
==========================================
  Files          26       27       +1     
  Lines        1021     1028       +7     
==========================================
+ Hits          857      864       +7     
  Misses        164      164