Save/restore Circuits to HDF5

[bendudson]

Circuits are saved to HDF5 files as an array of dtypes, which are created using Coil.dtype:
https://github.com/bendudson/freegs/blob/master/freegs/machine.py#L74
Unfortunately, circuits may include different types of coils, e.g. ShapedCoil and MultiCoil. This means that e.g. machine.MASTU can't be written to HDF5. Some possibilities:

1. Use groups to represent circuits. In principle circuits could contain other circuits, so groups could be nested
2. Define a coil dtype which is a superset of all the coil types which could be added. This would then not be future-proof, for adding new coil types, and I don't think would work with circuits which contain circuits.

[ZedThree]

Looking at how to convert to/from IMAS, there are two relevant data structures: pf_active and pf_passive. Note that the schemas on the website are not the most recent versions, so you might need to look through the actual schemas.

They're slightly different, pf_active has coils while pf_passive has loops, but they are both made up of elements:

Each PF coil is comprised of a number of cross-section elements described individually

And each element has area and geometry properties, with geometry being a structure itself, with a geometry_type property:

Type used to describe the element shape (1:'outline', 2:'rectangle', 3:'oblique', 4:'arcs of circle, 5: 'annulus', 6 : 'thick line')

Each of these corresponds to a separate structure with its own properties, e.g.:

 "pf_active.coil[:].element[:].geometry.outline": {
  "data_type": "STRUCTURE",
  "documentation": "Irregular outline of the element. Do NOT repeat the first point.",
 },
 "pf_active.coil[:].element[:].geometry.outline.r": {
  "coordinates": [
   "1...N"
  ],
  "data_type": "FLT_1D",
  "documentation": "Major radius",
  "type": "static",
  "units": "m"
 },
 "pf_active.coil[:].element[:].geometry.outline.z": {
  "coordinates": [
   "pf_active.coil[:].element[:].geometry.outline.r"
  ],
  "data_type": "FLT_1D",
  "documentation": "Height",
  "type": "static",
  "units": "m"
 },

There's definitely some correspondence between FreeGS coil types and IMAS geometry types, but maybe not one-to-one.