Add timers for each type of equation solve

[landinjm]

Add timer subsections for linear, nonlinear, and explicit, and aux solves (beyond the compute time in LHS & RHS) will be a useful diagnostic in assessing code performance & for preconditioning (if any).

[landinjm]

#285 Depending on how we end up changing the way users submit the terms for each field, we could potentially add timers for each field calculation.

I think this is valuable information in terms of debugging and optimization.