Exceptions: Add new UninitializedEntity exception
ImplContainer: Use new UninitializedEntity exception

Closes issue #134
[test] ImplContainer: Add test for UninitializedEntity exception
Merge pull request #137 from gicmo/uninitializedEntity

Add new UninitializedEntity exception and use in ImplContainer
removed setter of dataType in Property, getter returns Value.type
Docstrings in Property.hpp
Merge pull request #139 from jgrewe/data_type

Intermediate solution for DataType handling in Property

LGTM