Import structure is confusing

[rkingsbury]

The current import structure makes it impossible to import the top-level module, which may cause confusion for users not accustomed to the from X import Y pattern.

import membrane_toolkit

and subsequently access the core functions. e.g.

membrane_toolkit.core.donnan_equilibrium()

fails with

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'membrane_toolkit' has no attribute 'core'

Consider revising the import structure and/or using namespace packages instead