146 separate classification module from standard dependencies

[emlynjdavies]

This PR creates optional dependencies for tensorflow, which are only needed for the Classification module.

• Fixes Separate classification module from standard dependencies #146
• Start of what is needed for organize dependencies by groups #140 and How to include optional dependencies that are instrument specific? #14

Now, if the [steps.classify] step is not defined in the pipeline, particle classification is skipped and no probabilities are reported. This is implemented in a backwards-compatible way, and which allows pipelines to be run without a tensorflow installation when classification is not needed.

New installation option:

The installation of the classification dependencies can be done with:

poetry install --extras "classification"

or (for Apple silicon)

poetry install --extras "classification-arm64"

After deployment (merging), this will be:

pip install pyopia[classification]

or (for Apple silicon)

pip install pyopia[claassification-arm64]

Other changes

• Additional documentation improvements, and CITATION.cff are also included in this PR