jtot_hires is not always needed

[theo-brown]

jtot_hires is meant to be used when computing the psi field from prescribed currents.

As this is only done in some cases, jtot_hires should be an optional parameter of the Currents dataclass.

For example, the function _calculate_currents_from_psi is meant to compute self-consistent currents from the psi field, and therefore does not require the jtot_hires to be set.