Use a consistent cache directory

[constantinpape]

We currently use different cache directories for saving the models vs. saving the sample image data.

• The models use $HOME/.sam_models.
• The sample data uses $HOME/.cache/micro-sam

It would be good to be consistent here and I think $HOME/.cache/micro_sam is a good choice. For the models we could use $HOME/.cache/micro_sam/models.

What do you think @GenevieveBuckley ?

[GenevieveBuckley]

I say it's a good idea and I'm happy to do that.

[constantinpape]

Ok! The path for the downloads is currently set here: https://github.com/computational-cell-analytics/micro-sam/blob/master/micro_sam/util.py#L50

The intention behind this code was to enable changing the cache directory via the SAM_MODELS environment variable. But this is not documented anywhere so we can change it.

My suggestion would be that we introduce a new environment variable MICRO_SAM_CACHE instead that defaults to $HOME/.cache/micro_sam. We should keep the option to set a different cache path for systems where the storage in $HOME is limited (e.g. computer clusters).