Thick maps: how to perform integration along line of sight?

[nvaytet]

Thoughts from February meeting:

• add a function as a argument that will be applied
• also need weights, then integrate the weights and the emissivity, and divide map by another map which is line intergal of the weights
• you don't have to feed in the density, you can compute a new Array which represents emissivity (e.g. data['hydro']['density']**2) and then you only need the weight
• what about the dz factor along line of sight? need to apply it manually later?
• how do we handle units? right now, the mechanism that only does something if operation is sum is weird.