Add a magnitude postOperation to the surfaceFieldValue function object

Functionality to add/problem to solve

Sometimes I don't care whether the output from surfaceFieldValue is negative or positive (like when monitoring the sum of the flux). Currently the only postOperation in surfaceFieldValue that appears to be supported is sqrt. Adding other simple ones like mag could be useful.

Target audience

Anyone who wants the magnitude of the value calculated by surfaceFieldValue

Proposal

Add a mag postOperation to surfaceFieldValue

Funding

I'm not prepared to fund this directly, but my guess is this will be useful to others. It just seems odd that only one postOperation is supported.