Refactor Network step() function

[djsaunde]

Currently, the step() function in the Network class accepts arguments mode, inpts, time. If we are to separate out the training / testing of networks from the object definition, we should be rid of the mode parameter (which gives the train or test phase).

Here's what I'm thinking:

1. Remove the argument mode.
2. Always update Synapses which have STDP "enabled".
3. "Enable" STDP on learnable Synapses during training; disable during test.

The enabling of STDP could be a boolean class-level attribute in a generic Synapse function.