Add tokamak option psi_divide_twopi

[bendudson]

Some equilibrium codes write GEQDSK files with a factor of 2pi in the poloidal flux (lookin' at you, LIUQE), resulting in poloidal fields that are 2pi too large. If this option psi_divide_twopi is set to true then the input poloidal flux is divided by 2pi.

• Closes #xxxx
• Tests added
• Udated manual
• Updated doc/whats-new.md with a summary of the changes

[johnomotani]

I'm happy to leave as is.

If you have the time/energy for it though:

• There's a cocos argument to _geqdsk.read() that would do the dividing by 2pi already
• The tricky part is that we call _geqdsk.read() before the TokamakEquilibrium options have been read, so we can't conveniently use them to decide what arguments to pass to _geqdsk.read()
• If we moved basically all the code in the read_geqdsk() function inside the TokamakEquilibrium constructor (to be used if eg a geqdsk_filehandle is passed, leaving the way of constructing in case it's used somewhere), then we could use the TokamakEquilibrium settings.
• Maybe add a cocos setting instead of the psi_divide_twopi one?

[ZedThree]

Building on John's suggestion, it might be nice to do this automatically inside FreeQDSK (see freegs-plasma/FreeQDSK#9)