astropy: Have a counter in SkyCoord initializations and raise a warning in the event of a lot of scalar initializations

Idea from @astrofrog from the 2019 coordination meeting: have a “counter” in SkyCoord that keeps track of how many times a scalar SkyCoord it’s initialized and raises a warning along the lines of “Are you doing scalar initialization of SkyCoord in for loops? Maybe you should use vectors - see LINK”.

Suggestion from @perrygreenfield - is it really a “rate” (initializations / time) vs “total count” that we care about?

cc @adrn @taldcroft @perrygreenfield