rxd should not be imported unless it's at the request of the user

[ramcdougal]

Context

Overview of the issue

Opening ModelView with Python available requires numpy, which I think is because it loads the rxd module to look for reaction-diffusion dynamics. (rxd does require numpy.)

See https://www.neuron.yale.edu/phpBB/viewtopic.php?p=19899

Expected result/behavior

numpy should not be required for users only using HOC with no reaction-diffusion.

Possible fix

Importing rxd should set a "private" flag somewhere, probably in the top level neuron module, that ModelView can check for before attempting to load rxd itself.