blotting in lv3, flags for blotting + drizzling

[rchwn]

In #165 we added some lines at the end of Lv3Step to drizzle all of the individual exposures to the WCS of the mosaic.
This PR adds a flag to be able to skip that (since it produces a lot of files that not everyone wants), and it is now skipped by default.

Here we also add the ability to blot the aforementioned drizzled exposures back to the detector frame of some reference exposure. By default this is the first exposure in the list (alphabetically sorted). This requires drizzled exposures in order to work, and is turned off by default. The flag here is do_drizzle_and_blot.

End-to-end run on a test case with do_drizzle_and_blot=true worked for me. And do_drizzle worked before this update.

Example snippet of config.toml:

[parameters.lv3]
# do_drizzle = true # ignored if do_drizzle_and_blot = true
do_drizzle_and_blot = true
blot_ref_index = 0 
blot_fillval = np.nan

[thomaswilliamsastro]

Happy to be overridden on this, but I would rather have 2 parameters, do_drizzle and do_blot, where is do_blot is True when do_drizzle is False throws up a warning that do_blot needs to be set True. Just think that's neater

And please add some description to docs/steps/lv3.rst!!!

[rchwn]

@thomaswilliamsastro Done!