preprocess tested and ready to merge

[dyang37]

This is a PR to merge preprocess module into master branch. See below for preprocess API docstrings: mozilla.pdf

I tested on various NSI datasets to make sure that the default parameters work on all corner cases we have encountered so far (landscape vs. portrait mode, horizontal/vertical flip of the radiograph, etc.).

[dyang37]

• In docstrings, explain what blank_scan, dark_scan, obj_scan are.
• Change compute_sino_weight_from_scans to transmission_CT_preprocess.
• Change NSI_process_raw_scans to NSI_load_scans.
• Change NSI_read_params to NSI_load_params
• Need to rethink how weights is calculated in preprocess for NSI dataset.
• weight_type -> weight_type_ct.
• Options: square_root_weighting: $\sqrt{I/\bar{I}_0}$, where $\bar{I_0}$ is the averaged open beam value across all detector pixels. regular_weighting: $(I/\bar{I}_0)^\beta$ beam_hardening_weighting. A possibility for the future.

[dyang37]

Closing this PR for now. We can reopen this once we reach consensus on the interface design of preprocess module.