Blank electric field option for implicit solvers

[JustinRayAngus]

This is the first of two PRs adding the capability to not evolve ("blank") user-specified components of the electric field when using the electromagnetic implicit solvers. This PR introduces the basic functionality, while a follow-on PR will exploit the blanking options to improve the efficiency of the implicit solve.

In 1D geometry, this feature enables the electromagnetic implicit solvers to be used as the energy-conserving electrostatic model described in https://doi.org/10.1016/j.jcp.2011.05.031, in which the electric field is advanced via Ampère's law. This approach has been used in the following works:

• 10.1103/PhysRevE.104.055205
• https://doi.org/10.1103/pr7n-48wl
• https://doi.org/10.1103/zwjx-jbxl

In 2D XZ and RZ geometries, this feature can be used to disable evolution of the out-of-plane y-component of the electric field. This is suitable for problems with symmetries for which only in-plane currents are relevant (e.g. m=0 modes in a Z-pinch).

When symmetries permit field blanking, there is potential to significantly improve the efficiency of the implicit solvers. For example, when the y-component of the electric field can be ignored, the number of required mass matrices is reduced from 9 to 4: only sigma_xx, sigma_xz, sigma_zx, and sigma_zz are needed.

TODO list:

• Add CI test.

[dpgrote]

This should be indented.