Linear BMM notebook clean-up

[asemposki]

During the code review of Taweret for the v0.4 release of the BAND framework software, it was discovered that the notebooks for linear model mixing could use a lot of cleanup, mainly concerning the printing issues for the plots, interfacing with Pandas, and spell-checking. We also would like to investigate the nan results given by the bilby sampler and whether they are expected or not. This Issue will be for the Taweret team post-release of v0.4 of the BAND software.