Speed up network generation (#319)

* sped up tl.generate_network

turns out .groupby is a lot faster than trying to loop it like how it's currently working.

* shh

* Update _network.py

* ok this is the correct place to adjust

* Revert "Update _network.py"

This reverts commit 2a6ad82.

* does this speed up?

* add option to only generate expanded network

* Update api.rst

* Update _network.py

* unnecessary copy

* Update test_mouse.py

fix regression

* Update _network.py

* fix error