Moves stats imports from top-level __init__.py to stats folder

[ilumsden]

Previously, all of our stats functionality was exported through thicket/__init__.py. This goes against user expectations from tools like SciPy, and it lead to issues with the optional dependency on seaborn.

To fix this, I have moved all the stats re-exports into thicket/stats/__init__.py. So, now, to import e.g., mean, users would use one of the following:

# Option 1
from thicket.stats import mean
# Option 2
import thicket as th
th.stats.mean(...)

Additionally, this PR wraps all the "display" imports from stats in a try-except-else block that will only import the display functionality of stats when seaborn is present. If it is not present, users will get the following printout:

Seaborn not found, so skipping imports of plotting in thicket.stats
To enable this plotting, install seaborn or thicket[plotting]