Restructure code so that the functions take the polarization as a parameter.

Also remove prepare_fields and fields_at_prepared_positions, not really any usecases.
Closes: #3

Author: Mika Pflüger

README.md

@@ -10,5 +10,3 @@ The functions in this module are:
 * `fields_parallel`: As fields, but parallelized.
 * `fields_at_positions`: Calculate the transmission and reflection amplitudes at specific positions in the stack.
 * `fields_at_positions_parallel`: As fields_at_positions, but parallelized.
-* `prepare_fields`, `prepared_fields_at_positions`: Separately calculate the transmission and reflection amplitudes for each
-    layer in the stack and then at specific positions in the stack. (special use cases only)