Refactor filament data use

[connoramoreno]

Streamlines and reorganizes use of filament coordinate data as follows:

• Instantiates a new MagnetCoil class object as filament coordinate data becomes available in the MagnetSet._instantiate_coils method (formerly named MagnetSet._extract_filament_data), rather than store all filament data separately (and redundantly) in the MagnetSet class.
• For the purpose of filtering coils based on toroidal angle, moves the check of whether each coil lies within the model's toroidal extent to the check_coil_toroidal_extent method of the MagnetCoil class, rather than the _filter_coils method of the MagnetSet class (formerly named MagnetSet._filter_filaments). The MagnetCoil._check_coil_toroidal_extent method is called by the MagnetSet._filter_coils method to subsequently perform the actual filtering.
• Modifies storage of filament coordinates in the MagnetCoil class such that all coordinates are stored, and only downsampled during actual CAD solid generation. Note that the downsampled coordinates are not stored as part of any class.

Closes #47
Closes #149 (via PR #152)

[gonuke]

Two little code comments....

In a separate issue/PR we should discuss whether you check the extent of the filament data before building the coil?

[gonuke]

Thanks @connoramoreno