Constant tension TF coil shape procedure (#3640)

* Create constant_tension.py

First pass constant tension procedure.

* move staticmethod to private function

* pause

* errors

* clean up

* weird pytest error message

* need to sort out n_points

* working v test

* working tests

* working tests

* add test docstring

* remove plotting

* add working Ripple optimisation problem with discrete princeton D

* add example

* update docs

* move import

* minor

* add parameterisation tests

* docstrings

* documentation

* init fix and test

* put back docstring plot

* add call error and test

* __all__

* better transition to vertical

Author: CoronelBuendia