add warnings to inform that SRTomo is not real tomography

[rafaelmds]

Fixes #371

This PR includes warnings about the SRTomo class informing that is not a real tomography.

Checklist:

• Make tests for new code (at least 80% coverage)
• Create/update docstrings
• Include relevant equations and citations in docstrings
• Docstrings follow the style conventions
• Code follows PEP8 style conventions
• Code and docs have been spellchecked
• Include new dependencies in doc/install.rst, environment.yml, ci/requirements.txt.
• Documentation builds properly (run cd doc; make locally)
• Changelog entry (leave for last to avoid conflicts)

[leouieda]

@rafaelmds thanks for doing this! Merging.

[rafaelmds]

Hello Leo,

doesn't it need a changelog entry for v0.6?
I was waiting automatic tests to push that commit.

[leouieda]

Oops, sorry. That's OK, this is only a documentation change so no need for a changelog entry.