Forbid import when tensorflow is not installed

[willGraham01]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
See brainglobe/BrainGlobe#27 - this PR is for conda-forge compatibility reasons.

What does this PR do?
This PR forbids the import of cellfinder_core when tensorflow is not installed.

Note that we do not need to remove tensorflow from the pyproject.toml dependencies - these are read only by pip and not by conda.

References

Resolves step 1 of this issue.

How has this PR been tested?

Local testing of pip package install:

$ conda activate cellfinder
(cellfinder) $ pip install .
(cellfinder) $ python 
>> import cellfinder_core
# All good
>> exit()
(cellfinder) $ pip uninstall tensorflow
(cellfinder) $ python
>> import cellfinder_core
# Throws expected error
(cellfinder) $ pip install tensorflow
(cellfinder) $ python
>> import cellfinder_core
# All good again
>> exit()

Is this a breaking change?

No. New version, PyPI upload, and conda-forge recipe required though.

Does this PR require an update to the documentation?

Updates README to explain that the conda-install will not fetch tensorflow, this will have to be done by the user.

Requires a change on brainglobe-meta repository.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit