[Roadmap area] Parameter refactor

[brosaplanella]

Refactor parameter values to use the BPX format by default.
Using nested dictionaries allows cleaner definition of negative / positive and also different components of the battery.
This is a big breaking change so we should add a conversion class from legacy parameter sets to new ones, and legacy parameter sets should still work for a while.

Originally posted by @tinosulzer in #3839 (comment)

[rtimms]

FYI we (Ionworks) have a FUSE internship open for applications that is related to this, so we can support this over the summer

See https://www.linkedin.com/jobs/view/3851436140/

[valentinsulzer]

I think the main points to discuss are:

1. What to call the new parameters (stick close to pybamm or switch to bpx). Does the latest version of BPX allow us to specify all parameters, including miscellaneous ones? If so, I suggest we align ourselves completely with that, even if we don't totally agree with all the parameter naming choices. My only change would be that our specification should be python-based instead of json-based to allow for any arbitrary python functions / interpolants.
2. How to handle string names for the nested dictionaries (e.g. pybamm.Parameter("Negative::Particle size [m]"))
3. How to do the transition slowly / in a backwards compatible way

[valentinsulzer]

Also, when we do this, it would be great to do a deep review of existing parameter sets, including removing parameters that are not provided in the original reference (e.g SEI parameters for a performance-only model). See #3919 (review)