Updating dwarfs targets and rosters

[kadrlica]

This PR is to start the process of updating the default photometric properties for the dwarfs to reflect the newest measurements collected in the 2020 dwarf census (1912.03302). The big question for this PR is how we want to deal with changing the default values. Due to the library structure, a change to the ra,dec in the defaults will propagate to each of the other rosters that depends on the defaults to set these values. This will cause an unexpected change for users who will now see slight shifts in their satellite parameters without having updated anything.

There are two possible ways to address this (though neither has been done yet):

1. We could have every new target explicitly specify base: default2020 (the default is base: default). This will leave the default unchanged for the older targets.
2. We can move the old defaults to defaults2016 and have every old target explicitly derive from base: default2016.

It seems like we should probably start specifying the base explicitly in all new and old targets. Maybe this is a good time to make this change.

[kadrlica]

This PR is relevant for #19 and #11.

[kadrlica]

A new default keyword has been added to the creation of targets. This should allow the default set to be updated transparently.

[kadrlica]

@dimauromattia (the main user) says

I would move with option 1, in this way all the scripts and setup for the older analysis is unchanged. Instead, in future runs we have to specify whether we want to use the new default.

This sounds good to me, so let's move forward in that direction.