Cylindrical to Cartesian

[tomc271]

Added a new method CylindricalCoordinatesToCartesian() on TokamakOptions class, which returns Cartesian coordinates x, y, z (as Field3D objects) calculated from the cylindrical coordinates Rxy and Zxy (Field2D objects) together with nz equally spaced points for the values of the toroidal angle.

[ZedThree]

Looks like you just need to merge recent next in to fix the test failures

[dschwoerer]

Could you mention why do you need this? Is this only useful for you to cleanup the examples - if so could you demonstrate how that makes things easier? Or is this generally useful, in which case it probably should be documented somehow?