Add helper function to split run JSON into multiple smaller files

[ngreenwald]

Is your feature request related to a problem? Please describe.
Generating a single JSON to image and entire is useful to ensure proper randomization and avoid batch effects. However, it can be too long to image all at once. This means that the user either has to manually delete FOVs from the run which is slow, or edit the JSON computationally, which is error prone.

Describe the solution you'd like
A helper function to take in a JSON file and split it into smaller ones based on a) desired amount of time per run or b) desired number of FOVs per run

[ngreenwald]

[camisowers]

Closed with PR #145