Pytomography

* Monte Carlo Scatter correction has been implemented via use of the SIMIND Monte Carlo program. If users have SIMIND installed, they can use the functionality of the library
* Basic implementation of the StarGuide system matrix. So far only tested for Tc-99m (be careful with higher energy isotopes as there may be significant components of septal penetration and scatter not yet compensated for)
* Cardiac functionality for reorientation after reconstruction
* Other minor bug fixes

Minor update allowing for use of priors in multipeak reconstruction via update of the `ExtendedSystemMatrix`

* Bug fixes for reading SIMIND v8.0 data (collimator params now in mm) while still allowing for functionality with older simind data
* Add plot utilities for creating superimposed SPECT/CT and PET/CT plots
* Upgrade to readthedocs
* Other minor bug fixes

* Fix implementation of LogCosh prior

* Add support for advanced SPECT PSF modeling in conjunction with the [SPECTPSFToolbox](https://github.com/lukepolson/SPECTPSFToolbox)
* Changes to prior scaling in PET reconstruction that take into account sensitivity within the FOV
* Minor UI changes

Fix nograd bug