Unit test to check Battaglia profile against NFW profile

[evavagiakis]

@kbanker to work on unit test which checks the residuals from portion of Battaglia profile against NFW profile from existing function such as https://docs.astropy.org/en/stable/api/astropy.modeling.physical_models.NFW.html and ensures they are within (5-10?)%. We can determine the number together after plotting some examples and reviewing https://arxiv.org/pdf/1109.3711.pdf

[bnord]

some guidance for writing unit tests:
https://lsstdesc.org/pages/DESchool.html#MikeJarvis

[evavagiakis]

This unit test is stale and we will need to decide on new unit tests for our initial release. I'm closing the issue and have added a new one here #152 which describes adding a function to plot an NFW profile but decouples it from any unit test