Remove PythonAPI directory and most references

The link to the system installed/3rd party version of boost python
is now restored.

This required an additional helper function: isNone. boost python
only introduced object::is_none in version 1.43 so the helper
uses it if it is present but falls back to checking manually if not.

Refs #7607
Implement a RemoveConst return_value_policy.

Earlier boost versions, <= boost 1.41, have
problems registering conversions between const/non-const pointers
for exported Python types. The return value policy allows this
to be avoided by removing the const on return of an object to Python
without having to create another proxy function.

Const pointers as function arguments will still require proxy functions
though.
Refs #7607
Remove Python registration of pointer to const types.

Those methods that return pointers to const types now use the new
RemoveConst return_value policy.
Refs #7607
Remove unused headers.

Refs #7607
Rename downcast_returned_value to DowncastReturnedValue ...

to be consistent coding/naming guidelines.
Refs #7607
Merge branch 'feature/7607_remove_v1_python_api' into develop