Add BPX support

[BradyPlanden]

Feature description

This issue is for adding BPX support for import/export of parameter sets. Initial thoughts on the API:

To export:

bpx = pybop.BPX(results=results, parameter_set=parameter_set)
bpx.save(file='test_params.json')

To import:

parameter_set = pybop.BPX.import(file='test_params.json')

Motivation

No response

Possible implementation

No response

Additional context

No response