[ENH] Add a function to remove Median background per read

[stscicrawford]

Remove Median background per read from the IMA step. The algorithm is describe in the notebook here:
https://github.com/mackjenn/wfc3/blob/Nonlinear_bkg/nonlinear_bkg.ipynb

This would correct for Helium I emission (flat pedestal of extra signal) by subtract median per read and rerun calwf3 with ramp-fitting turned on. 

Requirements for completion:

• Documentation
• Functionality
• Testing

[mackjenn]

A new function has been written by @shannnnnyyy of the WFC3 team to flatten the IR ramps.
#36
https://github.com/shannnnnyyy/wfc3tools/blob/ir_tools/wfc3tools/wfc3ir_tools.py

We need to understand the process for merging it into wfc3tools, as we would like to advertise this tool for WFC3 users in the near future.

The notebook linked above has been modified to call this new function, and will be included in the ir_tools documentation. It's currently sitting here: /grp/hst/wfc3k/mack/python/wfc3tools/wfc3/test_irtools/flatten_ir_ramp.ipynb