Automate source wrap

[ArivoliR]

Description

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Fixes #1410

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[rtimms]

Thanks taking this on! For it to work properly we need to wrap all the terms that aren’t a spatial operator in source, eg see

PyBaMM/src/pybamm/models/submodels/thermal/pouch_cell/pouch_cell_2D_current_collectors.py

Line 89 in 9415db5

self.rhs = {

I’m not sure this is what your solution does. Can you add a test to show that pybamm.laplacian(a) + b gets turned into pybamm.laplacian(a) + pybamm.source(b) to show this works in the simplest case? Then we can go from there.

[ArivoliR]

Okay I've made a huge mistake clearly, will try to figure out if there's a way to handle all these cases by just modifying the wrap_source function that I've made. Have my final exams going on, will continue my attempt at solving this issue in a few weeks