support XYZCT images for TileDataset

[jacob-rosenthal]

Fixes #232 by adding support for 5-channel images (XYZCT) in TileDataset, and adds tests.

[codecov-commenter]

Codecov Report

Merging #233 (3cbf174) into dev (1308b58) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #233      +/-   ##
==========================================
+ Coverage   85.24%   85.30%   +0.05%     
==========================================
  Files          25       25              
  Lines        2535     2538       +3     
==========================================
+ Hits         2161     2165       +4     
+ Misses        374      373       -1     

Impacted Files	Coverage Δ
pathml/ml/dataset.py	97.56% <100.00%> (+0.19%)	⬆️
pathml/core/slide_data.py	85.71% <0.00%> (+0.47%)	⬆️

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