* On lines where the variable type is obvious from inspection, avoid repeating the type using auto.
* When possible use a ranged for loop instead of begin() and end() iterators
* Add always zoom invariant and always on top rendering to
ViewProviderDatumCS
* Add a new utility class AxisOrigin for more efficient axis rendering.
Also exposed to python.
* Change ViewProviderPlacement to use AxisOrigin for rendering.
A single instance of AxisOrigin is shared by all
ViewProviderPlacement. Selection context is used to distinguish among
different instances.
