Allow custom experiment steps

[valentinsulzer]

The logic for specifying how experiment steps work is currently in the Simulation class. We should move this to the Step classes, so that users can specify any custom behavior they want for which variable controls the step, and which variable(s) are used in the termination. This will have the added benefit of reducing some of the complexity of the simulation class.

This involves replacing any if statement that checks op.type with a call to a function in the step class, e.g. https://github.com/pybamm-team/PyBaMM/blob/develop/pybamm/simulation.py#L222

[valentinsulzer]

Adding custom events should be really easy and could be done first separately, we just need to implement a Step.events() function that returns the appropriate event(s)

PyBaMM/pybamm/simulation.py

Lines 279 to 284 in 7c15368

	new_model.events.append(
	pybamm.Event(
	"Current cut-off [A] [experiment]",
	abs(new_model.variables["Current [A]"]) - term["value"],
	)
	)

We could allow the termination argument of Step to take in this function directly

[valentinsulzer]

For the custom steps, there will be two separate cases:

1. Current is specified explicitly, either as a constant or as a function of other variables, user supplies the explicit expression
2. Current is an algebraic variable, user supplies the algebraic expression for the current variable

Current and C-rate falls into 1, everything else falls into 2.

We will need two different CustomStep classes (e.g. CustomStepExplicit, CustomStepAlgebraic`) with examples for each one.