Feat: possibly add non-dimension coordinates to the MDAEvent

[tlambert03]

Many libraries like xarray, netCDF, pandas, and GDAL have the concept of dimension coordinates and non-dimension coordinates, where dimension coords are used for label based indexing and alignment and non-dimension coords provide additional info that can be useful for plotting, etc...

when we iterate over grid positions, the MDAEvent.index attribute only gets a single p index, but it would be useful to also pass on row/column, etc... The same concept could arise in other cases, and it would be nice to have a place for that. Could be an additional field, or some sort of convention in index (like {'p': 4, '_r': 1, '_c': 1}