Module mappers

This module contains machinery for synchronizing models with various objects.
Various terms used in this module are defined here.

param: a data element of the type schrodinger.models.parameters.Param. Params
can be ints, bools, strings, etc., or more complex compound params that are
themselves composed of multiple params.

model: an object with one or more params, each representing some data elements
of the model. The model can by synchronized to a target object via a mapper.

target: a target is any object that we want to keep in sync with a model param.
Targets are generally GUI widgets like spinboxes or line edits, but can be a
variety of other things, such as a specific signal we want a model param to
listen to, or a pair of setter/getter functions to sync to a model param's
value. A target could also be something like a command line argument, such that
each command line argument corresponds to a different param in a model.

access: a particular way of interacting with a target. A target can have one or
more accesses - a setter, a getter, or a signal.

default access: certain target types will have default accesses defined in this
module. The default accesses for QLineEdit, for example, are: QLineEdit.text as
the getter, QLineEdit.setText as the setter, and QLineEdit.textChanged as the
signal.

mapper: a manager object that is responsible for model/target synchronization.

mapping: a defined association between a target object and a model param. Note
that the mapping is always between a specific target instance (for example a
checkbox instance), and a model *class* param (ex. MyModel.myboolparam, where
MyModel is the class). By making the association with the model's class rather
than a model instance, the mapper is able to switch between different instances
of the same model. Consider, for example,

    A model class Person, with params name and age
    A GUI panel with panel.name_le and panel.age_sb
    Mappings:
        panel.name_le -> Person.name
        panel.age_sb -> Person.age
    Model instances amy, bob, and charlie

We can now user mapper.setModel to switch between model instances, and the GUI
state will change accordingly.