Type annotations, doc-string re-formatting, code cleanup.

[djsaunde]

Mostly cosmetic changes or refactoring.

Type annotations are useful because

1. IDEs can catch errors before you run the code
2. mypy can be used to catch type errors as well
3. Makes documentation easier (don't have to annotate argument types in doc strings)

That said, type annotation doesn't have to be done for every function.

If you use the PyCharm IDE, type annotations and auto-documentation works really well.

I also changed many imports of the form

from bindsnet.network import *

to be more explicit, i.e.,

from bindsnet.network import Network

so we're not reloading modules all over the place.