Pytomography

* Minor fixes to a few bugs in previous versions
* A couple new reconstruction algorithms which will be put in tutorials in newer versions
* Ability to include scatter variance estimate in uncertainty calculations
* A couple updates to the tutorials

- Fix bug where `parallelproj` was imported for regular SPECT system matrix

**Note: this update will introduce breaking changes to past code. Be sure to check out the new tutorials for updates. In particular, we have now incorporated ``Likelihood`` functions in reconstruction**.

* Statistical iterative reconstructions have now been completely rewritten and placed in `PreconditionedGradientAscent` algorithms. These work similar to before, and also can be used to estimate uncertainty in various VOIs
* New submodule `Likelihoods` now required for reconstruction. This allows different imaging systems to be implemented
* Minor changes to IO / other.

- Fix bug where `from __future__ import annotations` was not present in kem_system_matrix file.

- 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.