Change name of emission wavelength on the OpticalChannel

[CodyCBakerPhD]

A big confusion/complaint we recieve, not just about how the indicator is a property of the ImagingPlane (this is currently fixed in ndx-microscopy design) but also how the emission_lambda is (a) not a scalar and (b) a spectrum known from the indicator

Instead, it seems that the intention of this value is to represent the 'median wavelength selected by the optical filter'; I've also needed or wanted to attach bounds on that filtering as indicated by the methods section of multiple papers

As such I'd propose changing the name to something like filter_wavelength_median and also add filter_wavelength_range (or @alessandratrapani preferred 'bandwidth') which is a [2] (float) with both properties in units of nanometers