Allow negative phi angles for CylindricalMesh (and SphericalMesh)

[paulromano]

Right now, the CylindricalMesh class constrains the phi argument to be between 0 and 2π. If the user wants to tally a segment in the phi angle that covers ϕ=0, it's currently not possible. For example, the phi bin [-π/4, π/4] would be rejected because the first angle is less than 0. In addition, the bin [7π/4, π/4] wouldn't work because it expects it to be monotonically increasing.

To allow this use case, we could potentially allow any set of monotonically increasing angles within [-2π, 2π] as long as the min and max values are within 2π of each other. So for example, the range could be between [-2π, 0], [-π, π], [-π/2, 3π/2], etc. This would require changes on the C++ side to normalize the phi range to the range that the user specified (currently it always forces values onto the interval [0, 2π]).

Thanks to @spasmann for bring this up to me.

[paulromano]

Here is one relevant snippet of code that would need to be updated:

openmc/src/mesh.cpp

Lines 1201 to 1202 in 1645e3b

	if (mapped_r[1] < 0)
	mapped_r[1] += 2 * M_PI;

We would also need to change the checks on allowed values, and when the mesh is considered to cover the full phi range:

openmc/src/mesh.cpp

Lines 1383 to 1395 in 1645e3b

	if (grid_[1].front() < 0.0) {
	set_errmsg("phi-grid for "
	"cylindrical meshes must start at phi >= 0.");
	return OPENMC_E_INVALID_ARGUMENT;
	}
	if (grid_[1].back() > 2.0 * PI) {
	set_errmsg("phi-grids for "
	"cylindrical meshes must end with theta <= 2*pi.");
	return OPENMC_E_INVALID_ARGUMENT;
	}
	full_phi_ = (grid_[1].front() == 0.0) && (grid_[1].back() == 2.0 * PI);