Fix validation and checkpointing interval

[bittremieux]

The option to configure the interval for validation and checkpointing has been changed:

• Config option every_n_train_steps has been renamed to val_check_interval in accordance to the corresponding Pytorch Lightning parameter.
• This now determines the number of training steps across epochs instead of within a single epoch. I.e. previously if there were 75,000 batches in the training data, validation would run at epoch=0,steps=50k; epoch=1,steps=50k; epoch=2,steps=50k; ...; whereas now it will run at epoch=0,steps=50k; epoch=1,steps=25k; epoch=1,steps=75k; ...

Fixes #211.

[codecov]

Codecov Report

Merging #224 (04fc7de) into dev (ad48a09) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #224   +/-   ##
=======================================
  Coverage   88.87%   88.87%           
=======================================
  Files          12       12           
  Lines         899      899           
=======================================
  Hits          799      799           
  Misses        100      100           

Files Changed	Coverage Δ
casanovo/config.py	87.80% <ø> (ø)
casanovo/denovo/model_runner.py	87.59% <ø> (ø)

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[VarunAnanth2003]

CodeCov coverage did not change and all tests passed. Looked at code and no changes will break anything. Good to go