Add `WaveForm` and `Enum` dataypes

[evvaaaa]

WaveForm

Will be either a Sequence[T] where T is float, int, or str, or a numpy array with a dtype. This will not be an arg to the dataclass (not optional).

There will be a length on the datatype set to some sane default, since this is required by the EPICS and tango backends.

Enum

Will take a python enum.Enum type. When putting to the attribute you put the enum member not a string/int value itself. We will check on initialisation that all the values are either string or int.

We should keep allowed values for checking on Attribute.set, but I don't think that the current system of making and mbb record if it's a string with allowed values is as nice as having a dedicated Enum datatype that can be altered to whatever control system's enum definition.

Changes which will be required across datatypes.

DataType will now be given two additional attributes:

• cast(T): For casting to a datatype which can be sent over the control system. It will also do some validation (i.e checking that an Int value doesn't exceed the maximum defined in the DataType).

WaveForm(Sequence[str]).cast(["a"]) == np.array(["a"], dtype="S40")
Enum(SomeEnum).cast(SomeEnum.member) -> "TheStringValueOfTheMember"
# Fails so that we don't have an internal value of the `Attribute` out of sync with the softioc.
Int(max=5).cast(6)

• @property\intitial_value(): We can't rely on Attr._datatype.dtype() for an initial value anymore since the structured numpy array's has to be initialised with shape (length in our case) and dtype=. For enums this is true too. Therefore, we will use DataType.initial_value for the default initial value on attributes. This can be defaulted to self.dtype() in the Attribute base class.

[GDYendell]

I think cast should be validate using the mechanism suggested here.

[GDYendell]

We should keep allowed values for checking on Attribute.set

Can we just do self.dtype.validate(value) instead of keeping allowed_values on Attribute? Actually, I don't think it even validates against allowed_values currently.

[evvaaaa]

I think cast should be validate using the mechanism suggested here.

Okay, I'll move it to that PR.

Can we just do self.dtype.validate(value) instead of keeping allowed_values on Attribute? Actually, I don't think it even validates against allowed_values currently.

I'll make it so that an enum type which exceeds the epics limit for options comes out as a builder.string in the backend.