loadOmero() improvements (rebased onto develop)



This is the same as gh-949 but rebased onto develop.

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This PR:
- fixes a bug in `unloadOmero()` (jar removal) and implement unit tests to check Matlab/Java path clearing 
- decouples `loadOmero()` from `connectOmero()` and improves the headers to better expose the various signatures
- allow to pass the path to a configuration file directly to `loadOmero()`
- fixes the ``ICE_CONFIG`` reading for `loadOmero()`
- add various `tearDown()` methods in unit tests to allow TestUnloadOmero to be called safely