Better encoding of what's imaged in each channel

[vreuter]

Right now, the parameters config file has hooks for specifying which channel's nuclei and which channel is FISH. But with #337 , we'd like a way to encode what's imaged in each channel in a more flexible way.

[vreuter]

Here's a snippet from ZARR metadata (.zattrs)...

    "metadata": {
        "channel_0": {
            "emissionLambdaNm": 450.5,
            "excitationLambdaNm": 365.0,
            "name": "DAPI"
        },
        "channel_1": {
            "emissionLambdaNm": 630.0,
            "excitationLambdaNm": 561.0,
            "name": "Red"
        },
       ...

So, the user could map channel index (0, 1, etc.) to a biological entity like "nucleus" or "fiducials", or could perhaps map "DAPI", "Red", etc. to those values. Need to think on this. Indices may be simpler since o/w the user would need to know what was captured and stored in the raw Nikon (or other camera) format.