Standardize "map_x" and "map_y" variable names for hyperspectral data

[stuart-cls]

I see this having two parts,

• Hyperspectra widget strongly prefers "map_x" and "map_y" on new context, even if they aren't the first two meta variables
  • This prevents the poor experience of opening hyperspectral data and getting the "map_x" vs "map_x" plot or other weird map
• Check through the FileFormat(s) and other widgets and make a single point of truth for map_x/y variable creation.

I think we're 90% there since this happens less and less to me but let's finish it off.

[stuart-cls]

This is done in #654 / continued in #750