quantiles arg in dscim.diagnostics.compare_sccs.compare_sccs() has a mutable default argument

`dscim.diagnostics. compare_sccs.compare_sccs()`'s `quantiles` arg has a mutable default argument. This is [a common gotcha](https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments), we might want to refactor this to avoid bugs.

https://github.com/ClimateImpactLab/dscim/blob/8a870c3e2e16fbc3077f33972c5ef59a9278bb7e/src/dscim/diagnostics/compare_sccs.py#L7