Added details to VmecInput documentation

FORTRAN_MISSING_VARIABLES moved to test cases (#311)

Automatically iterate over model_fields (which does not include any extra fields when present) when converting between cpp and json.

Adding new output quantities is significantly easier now and only requires adding the field to `VmecWOut` on the Python side. Serialization and deserialization will then immediately pick up on it.

Pass Magnetic response table to Python layer by reference

HDF5 compatibility with old versions

Default initializer for VmecInput (reads values from VmecINDATAPyWrap…

…per) (#260)

### What changed?

- Added a custom `__init__` method to `VmecInput` class that initializes default values from the C++ `VmecINDATAPyWrapper` class when no arguments are provided

### Why make this change?
**Pro:**
- Enables a "pure Python workflow", where the user doesn't have to load any input file. `VmecInput` can be comfortably constructed in a script, without providing explicit defaults for **all quantities**.

**Con:**
- Defaults aren't clearly visible for Python users without constructing and inspecting the object.
- only passing a few arguments and defaulting the rest isn't supported, which may be desirable too.

Alternative: Explicitly default all fields to the same as VmecINDATAPyWrapper

Trigger docker action on release

create venv after checkout

Remove HDF5 unit test (deprecated)

GCC already includes OpenMP headers on manylinux

Made lasym specific terms std::optional and Optional in Python

Update __init__.py

Apply suggestions from code review