`write_vmec_input` will create a vmec input file that `Equilibrium.from_input_file` will not correctly handle

[dpanici]

We assume the current profile in vmec inputs is current derivative, yet when we write a VMEC input file we save as power_series_I which is the current, so DESC cannot correctly read the vmec input file we make.

We probably should just make the input reader support current profiles from VMEC instead of just current deriv profiles (at least power_series_I )

[dpanici]

Add this as a test (just write/read)

[dpanici]

handle in #1816