Provide method to access individual external override settings

The framework loads the external override settings and uses them in any configuration parameter that has an external override name attached, Users have asked for the ability to access these values directly without the indirection of configuration parameter entries in descriptors. Currently the complete Settings object that holds all the external override settings loaded by the framework is accessible via UimaContextAdmin.
An improvement would be to allow individual values to be read using a method in the UimaContext interface, perhaps:

String getExternalOverride(String name)
String[] getExternalOverrideArray{String name)