Define a specific supported equation set for SPMe

[ejfdickinson]

Add to the BPX standard a specifically defined equation set for "SPMe", within which the BPX parameters are used.

This will require some discussion on whether the PyBaMM implementation is appropriate, or if the canonical SPMe defined by Marquis 2019 should be used. See also pybamm-team/PyBaMM#4145

[valentinsulzer]

I would push towards using the PyBaMM implementation (with better documentation) as this is the more "practically accurate" version, compared to the "mathematically accurate" version from Marquis