Create "Properly Lumped" thermal model

[aabills]

Description

This would implement an alternative battery model where the heat capacity is given as a lumped parameter rather than for each cell component.

Motivation

While the lumped thermal model in PyBaMM is physically informative and accurate, it poses a problem for practicing battery modelers who most of the time do not have detailed thermal characterization of each component of the cell.

Possible Implementation

Rather than implementing a new model, I propose that we just add an option "use lumped thermal capacity" which is default "false", and just alter the existing lumped submodel code.

Additional context

No response

[aabills]

This doesn't seem like a high priority, and I don't like doing double work, so I'm going to do this after #4594 is wrapped up