extensibility modifications

These changes are to allow the elda-extension module (https://github.com/rwalpole/elda-extension) to continue working with the latest version of Elda. The main changes are access modifications to two methods to allow access from subclasses and some dependency modifications to allow the extension module to take dependency versions etc. directly from the elda pom.xml. This includes adding a dependencyManagement block to the project pom.xml and moving some specific dependencies of elda-lda from the project pom to the elda-lda pom.