Update install instructions and environment .yml

[lauramurgatroyd]

Describe your changes

Updates UI env.yml to also install cilviewer. This is needed because some of the dependencies require specific python versions so installing cilviewer and then the additional dependencies on top may cause it to fail

Describe any testing you have performed

Consider adding example code to examples
Tested creating an environment using the environment file on windows with conda.

Link relevant issues

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the CIL developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'waiting for review'

Contribution Notes

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CILViewer (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties

Qt contributions should follow Qt naming conventions i.e. camelCase method names.

VTK contributions should follow VTK naming conventions i.e. PascalCase method names.

[paskino]

Maybe in the future we could host the yaml file as we do for CIL, https://github.com/TomographicImaging/CIL#create-a-conda-environment-with-cil