add functions for making detector-centered coordinate system.

[yoshinori-0778]

This PR adds functions for making maps in detector-centered coordinate system that is useful for beam characterization.
I confirmed get_each_detcen_P and calc_detcen_coadd_maps woking properly with Jupiter observation (after pre-process applied).
Current version need to execute get_each_detcen_P every single detector and then coadded them.
In addition, prior to get_each_detcen_P planet az/el needs to be calculated.
It took roughly 3min for 148 detectors with map size of 2deg and map resolution of 2arcmin.

[yoshinori-0778]

@mhasself Could you review this PR? Once this is merged, I plan to work to implement the unified planet mapmaker that I curretly developped in pwg-script into sotodlib.

[yoshinori-0778]

@mhasself
I ended up adding just get_each_detcen_P for now because the others are needed for making maps and I think it would be a good time to add them when I try to add unified map-maker.
I changed place of it from new file to planet.py. I think this place makes more sense.
For the question when we discussed, cdet > 0 for current codes that I am using, so X-axis is RA type.

I am also working on improving unified map maker in pwg-script. Once the improvement get settles, I would like to make a PR for them.
How does this strategy sounds?