Multiple variation/textcase for referencing SEI

[jeromtom]

SEI abbreviation for Solid Electrolyte Interphase in the source code (mostly in pybamm\models\submodels\interface\sei) is referenced by n_SEI and L_sei we need to standardize the case for abbreviations across the code base.

[jeromtom]

Related to #2333 and #2717.