Break formatting code into formatting.py

[jagerber48]

Move the code for formatting ufloat into its own module: formatting.py.

I made two tiny moves beyond what I would call pure copy and paste:

1. I always define the isinfinite method and never try to import it from math.
2. I copy and pasted the deprecation function from core.py into formatting.py to avoid dealing with a circular import. The proper way to do this would be to place deprecation() into a common utilities.py or similar that both core.py and formatting.py could import from. But I didn't want to wade into that move in this PR.

[newville]

I am not opposed to this change. I think it can and should wait until after the next release.

[andrewgsavage]

Would you like to revive this?

[jagerber48]

Yes, I would like to revive this when I have time. I am very busy right now and likely won't have times to get into the details on this for at least a couple of weeks. If anyone else would like to revive it in the meantime instead they should feel free.