Rename cellfinder CLI

[adamltyson]

To complete the naming simplification, what is currently the cellfinder cli tool (i.e. cellfinder & brainreg) should be renamed. This may also be good time to decide on how the cli tools should be used. I was thinking we could move towards a general brainglobe cli, perhaps defined in the metapackage, e.g.:

• To download an atlas - brainglobe atlas_install allen_mouse_10um
• To run the currentl cellfinder cli - brainglobe wholebrain input_data output_dir

Alternatively, we could just rename the current cli to something like brainglobe_wholebrain_cell and deprecate the current cellfinder cli.

[adamltyson]

Currently brainmapper is the best name we can come up with. Rather than long, descriptive CLI names, we're thinking of coming up with the best snappy name we can for each one and make sure the docs are clear what they do

[adamltyson]

This has now been completed. The tool is called brainmapper, but otherwise nothing else has changed.