Undefined variable when extracting sensivity maps

[Daval-G]

I was trying to run the example "SelfCalibrationReconstruction_Cartesian.py" but changed the Fourier operator (and the data accordingly) to a Stacked3DNFFT to test it.

The reconstructor raised an error because the variable z_sample_loc was not defined in:

pysap-mri/mri/reconstructors/utils/extract_sensitivity_maps.py

Line 166 in c39764b

z_samples=z_sample_loc,

It may be related to a previous issue: #62

[chaithyagr]

This seems to be an atifact of moving to new functions for accomodating variable kspace z lines in #58
I will get a PR for fixing this and #72 .
Also, I will add tests to ensure we dont miss this in future thereby increasing coverage also

[chaithyagr]

Fixed with #76