Vector fields docs

[smartalecH]

Would be good to add a simple page in the docs that describe how fmmax computes vector fields. The paper describes this to some detail, but even since then, fmmax has changed it's approach.

Some interesting details include...

• FOMs used
• Comparisons of methods (especially compared to their FOURIER counterparts)
• When to use some vs others

We can start will a "brain dump" (it doesn't have to sound like a paper). But documenting this will help us keep things consistent.

[smartalecH]

Here's some good info:

fmmax/src/fmmax/vector.py

Lines 113 to 130 in c6cd855

	The calculation finds the minimum of a quadratic loss function using a single
	Newton iteration. Rather than optimizing the real-space tangent field, the
	Fourier coefficients are directly optimized.
	The tangent field has several properties or invariances:
	- The tangent field is independent of the scale of the unit cell; if the
	unit cell is uniformly scaled (e.g. by changing units from nm to microns),
	the vector field is unchanged.
	- The tangent field for a supercell (containing e.g. 2x2 unit cells) is
	identical to that of a single unit cell, so long as the number of terms in
	the Fourier expansion is increased correspondingly. Note that this means that
	the tangent field depends upon the number of terms in the Fourier expansion.
	- The tangent field is independent of the resolution of the discretized unit
	cell. That is, whether the permittivity distribution is specified with a
	`(100, 100)` or `(200, 200)` shaped array has no impact on the resulting field.
	- The tangent field is independent of the amplitude of the array from which it is
	obtained, e.g. the permittivity contrast.