restructured "expression ops" examples:
* moved to package net.imglib2.ops.expression
* moved static interfaces and classes to separate files.
The structure is now as follows:
...ops.expression has the main interfaces and abstract base classes.
...ops.expression.ops has example expression implementations and the Ops class with static creator methods.
...ops.expression.examples has usage examples.
Merge remote-tracking branch 'origin/roi-experiments' into roi-experiments