Introduce TransversePolarization

[jagot]

At the moment, we have the following polarization states:

ElectricFields.jl/src/field_types.jl

Lines 3 to 5 in 6fd3802

	abstract type Polarization end
	struct LinearPolarization <: Polarization end
	struct ArbitraryPolarization <: Polarization end

Maybe it would make sense to introduce the following type:

struct TransversePolarization <: Polarization end

to distinguish between the commonly used transversely polarized fields (circular, elliptical, &c) from the truly arbitrarily polarized fields, i.e. those with a polarization component along longitudinal direction.