Add multithreading support in the electric field calculation

[fhagemann]

Not sure why we never did this before, but the electric field calculation did not support multithreading. Adding it can result in a significant speedup.

Example:

using SolidStateDetectors, Unitful
sim = Simulation(SSD_examples[:CGD])
calculate_electric_potential!(sim, max_tick_distance = 0.05u"mm",
    max_n_iterations = 1, refinement_limits = Float64[])

Simulation: ExampleCuboid
Electric Potential Calculation
Bias voltage: 2000.0 V
Precision: Float32
Device: CPU
Max. CPU Threads: 64
Coordinate system: Cartesian
N Refinements: -> 0
Convergence limit: 1.0e-7  => 0.0002 V
Initial grid size: (374, 366, 370)

Before this PR:

@time calculate_electric_field!(sim)

  6.976945 seconds (16 allocations: 579.623 MiB, 3.73% gc time)

After this PR (using 64 threads)

@time calculate_electric_field!(sim)

  0.304666 seconds (277 allocations: 579.653 MiB, 22.28% gc time)

@oschulz Not sure if the increase in allocations is something to worry about or not, but it is definitely faster now.

[fhagemann]

Also, I removed fieldvector_coordinates from get_electric_field_from_potential, because I cannot think of a scenario where this should NOT be wanted..

Maybe this function can even be mutating to avoid allocating memory.