Add a private interface to manage the configuration of the JVM through Jpype.

Pulls out some of the code from the PyJapc initialiser so that it is possible to easily subclass PyJapc to configure the JVM before any JAPC classes are loaded. This is particularly useful for setting systemProperties, and thus closes https://issues.cern.ch/browse/ACCPY-273.

I haven't added any tests or documentation as this is entirely private. Users may choose to benefit from this interface, but I only see it as a temporary solution to the problem. The correct solution probably involves users creating their own cmmnbuild_dep_manager instance and making that available to the Japc API constructor.