Add electric field vector function to Plasma object

[vsnever]

It is known that in some cases the radial electric field can affect MSE measurements. Ideally, the MSE model in Cherab should take into account static electric field, E, in addition to the Lorentz v⨉B term.

I propose adding the e_field property to the Plasma object by analogy with the b_field.

[Mateasek]

Closed with #466