[BUG] Writing custom config is not possible on HPC with shared installation of BrainGlobe

[Alessandro Felder]

Describe the bug
The default paths returned in https://github.com/brainglobe/cellfinder/blob/main/cellfinder/core/tools/source_files.py are not (necessarily) writeable by users (for good reason). It's generally a bad idea for users to write into the package files. The default should presumably point somewhere else.

To Reproduce
Run brainmapper tool on HPC as user (with no write permissions to BrainGlobe module!)

Expected behaviour
It is possible for users to write custom cellfinder configurations.