Signal:: `unit()`, `min()`, `max()` and `scale()`

[joelle-o-world]

Add meta data function to describe the units of a signal, its minimum/maximum value and its scale (ie "linear", "logarithmic")

This is going to require more thought

SignalReaders will also have matching methods, perhaps called expectedUnit() etc

For some processes (eg Sum) the unit/min/max/scale will be computed from the inputs.

Definitely try to make this a separate base class layer.