Description of PSI field map format incomplete

The description of the PSI field map format is incomplete. Only part of the header is described, but the actual values are not described.

For each radial value not only the field values, but also the first, second and third order derivatives wrt theta are read in. However, it seems that in the code the values of these derivatives are not used and instead are recalculated from the field values.

cc: @adelmann