Removed the interpolate() method from PropertyConverter.

In future PropertyConverter will be package protected and deal only with type
conversions.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1515444 13f79535-47bb-0310-9956-ffa450edef68
Added some generic get methods to the ImmutableConfiguration interface.

This is related to [CONFIGURATION-508]. These methods are a generic form of
specialized data type conversion methods. Added corresponding (non functional)
dummy implementations to AbstractConfiguration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1515445 13f79535-47bb-0310-9956-ffa450edef68