Select channels based on their labels

[cboulay]

I've had a request from a user to be able to select elements along a CoordinateAxis axis (i.e., "ch" axis) based on string pattern matching against its .data field.

Whether it's in Slicer or another node, the design might involve:

• expression: str | tuple[str, ...] parameter -- one or more regex expressions that the channel labels are matched against.
• During "reset" (i.e., on first chunk or when the input metadata has changed to trigger a reset), the integer indices of channels that match the expression(s) are calculated (and concatenated) and kept as a state variable.
• On every chunk, we simply use the integer indices.