resize individual images within NPZ

[ngreenwald]

The current version of build.py accepts a single resize parameter for an entire NPZ of images. However, if the image data within an NPZ is heterogenous, there may be different resize values that are appropriate for each image within that NPZ file.

• This PR adds the option to specify a separate resize value for each image in an NPZ in the combine_npz_files function.

• It pulls out the core resizing and cropping functionality from combine_npz_files into a new helper function

• It adds a compute_cell_size function which calculates the mean cell size within an image, which can be used to generate those resize values.

[willgraf]

Per our discussion, some small changes to clarify the tests and simplify/improve the compute_cell_sizes code.

[ngreenwald]

• new implementation of compute_cell_sizes which collects list of lists, and then determines how to calculate average (this looks so much cleaner!!)

• Comments on hardcoded values in testing functions to explain rationale

• check that error is raised for invalid resize_ratios