Enable 2D processed variables for arbitrary domains

[brosaplanella]

Description

At the moment, 2D processed variables use a series of if statements to check the domains on which to plot things, which end throwing an error if none of the cases is recognised.

PyBaMM/pybamm/solvers/processed_variable.py

Lines 288 to 329 in 695917e

	# Process r-x, x-z, r-R, R-x, or R-z
	if self.domain[0].endswith("particle") and self.domains["secondary"][
	0
	].endswith("electrode"):
	self.first_dimension = "r"
	self.second_dimension = "x"
	self.r_sol = first_dim_pts
	self.x_sol = second_dim_pts
	elif self.domain[0] in [
	"negative electrode",
	"separator",
	"positive electrode",
	] and self.domains["secondary"] == ["current collector"]:
	self.first_dimension = "x"
	self.second_dimension = "z"
	self.x_sol = first_dim_pts
	self.z_sol = second_dim_pts
	elif self.domain[0].endswith("particle") and self.domains["secondary"][
	0
	].endswith("particle size"):
	self.first_dimension = "r"
	self.second_dimension = "R"
	self.r_sol = first_dim_pts
	self.R_sol = second_dim_pts
	elif self.domain[0].endswith("particle size") and self.domains["secondary"][
	0
	].endswith("electrode"):
	self.first_dimension = "R"
	self.second_dimension = "x"
	self.R_sol = first_dim_pts
	self.x_sol = second_dim_pts
	elif self.domain[0].endswith("particle size") and self.domains["secondary"] == [
	"current collector"
	]:
	self.first_dimension = "R"
	self.second_dimension = "z"
	self.R_sol = first_dim_pts
	self.z_sol = second_dim_pts
	else: # pragma: no cover
	raise pybamm.DomainError(
	f"Cannot process 2D object with domains '{self.domains}'."
	)

We should enable a generic case where, if none of the above, the model is plotted taking the secondary domain as horizontal axis and the primary domain as vertical axis (to match the batteries case particle-electrode). Potentially, we could let the user modify this.

Motivation

This would enable running non-battery models in PyBaMM and plotting them.

Possible Implementation

Change current else statement to assign self.first_dimension and self.second_dimension from primary and secondary domain variables. It seems r_sol, x_sol et al are only used in testing.

Additional context

No response

[brosaplanella]

Similarly, in the 1D case, there's already an else case for generic models, even though it defines the first_dimension as "x" so it should probably also take it from the domain variable.

PyBaMM/pybamm/solvers/processed_variable.py

Lines 191 to 193 in 695917e

	else:
	self.first_dimension = "x"
	self.x_sol = space