Allow for changing of `Waveform` shape

In https://github.com/DiamondLightSource/FastCS/pull/119, we create an `NTArray` for `Waveform` values when using the p4p transport.

We would like to use a standard single dimensional array, e.g `NTScalar("ai")` when a shape of `(n,)` is given, and an `NTArray` when more than one dimension is given.

Once https://github.com/DiamondLightSource/FastCS/issues/111 is completed, we can change the shape on the `Waveform` and the p4p transport should swap between these types.