Pytomography

- support for opening GE projection files (before there was a bug)
- fix error in first two tutorials (SPECT simind and SPECT dicom)

Added discourse information on documentation website.

* Fix bug where SPECT modules couldn't be imported due to not having `parallelproj` installed
* Updates to motion SPECT with bug fixes
* Update to reconstruction algorithms which now check for required functionality in system matrices

- add link the DIPRecon repository in the PET examples

PET listmode and time-of-flight reconstruction is now here! This update adds
- PET system modeling of listmode and time of flight data. Options for reconstruction on GATE and PETSIRD data
- Deep Image Prior reconstruction algorithm
- Major restructuring of reconstruction algorithms to be modality independent

Updates to KEM to support kernel on GPU the whole time
Updates to PSF modeling to allow for arbitrary PSFs to be used in high energy SPECT
Allow precompute normalization factors in recon algorithms to make faster
Other minor changes