Add unit check for TOD operations

[mj-gomes]

This is an idea from @silviamich and me after we faced issues with units in tod operations.

We could force all tod computations (scan_map, fill_tod_with_hwp_harmonics, add_2f, add_non_linearity, etc.) to have compatible units. I am thinking of something similar (or exactly the same) as what is done in the map operations currently. This would avoid e.g. adding differential emissions in Kelvins to a TOD in spectral radiance units. Every operation changing the tod of an observation would start by passing a unit check.

We can discuss this better in the next meeting, meanwhile I will try to come up with a draft PR to do this.

[mj-gomes]

Would it be useful to have something like pint (possibly our own tailored version) that would associate every variable with a unit and automatically keep the dimensional coherence?

As we can see in some of the pint examples, if we multiply some variable which is, let's say in W.sr-1.m-2 with one in m2 it will return the result in W.sr-1, maintaining coherence. And if then we try to sum something with different units, it returns an error. Likewise, if we sum meters with centimeters, it does the automatic conversion and returns the result in meters.

Something like pint would be amazing to ensure correctness everywhere. For example, it took me some time to debug something which was basically due to a normalization factor that I had forgotten, which was making the units incompatible between two variables being summed, and the result was very incorrect. If we had something like pint, I would have gotten immediately an error message saying that I am not summing two things with the same units, because the normalization operation would automatically include the correspondent change in dimensions (I think this is really the most important part, as in the input sky module, for example, we have unit compatibility check, but we must define the units ourselves and I believe they do not update automatically with the operations). I believe this dimension coherence across all lbs would be very useful.

@ziotom78 @paganol, and others, what do you think? If you agree that this could be a good option, would you say it is better to develop our own pint-like module, or could we use pint (I believe pint allows us to define new units that may not yet be present in there, and it seems to be a pretty much alive and up-to-date package).