changing I3_WORK to I3_SRC and I3_BUILD

[dule]

With the current cmake compiling tool, we no longer have just the one main work directory (I3_WORK), but two relevant directories: source code dir (I3_SRC) and build dir (I3_BUILD).
Some components of the code still expect I3_WORK, even though this is obsolete now.
I created branches of both cmake and ithon called "src-build", which have these relevant changes.
These branches should be used in a release of offline-software and also merged with the trunk at some point.

With I3_WORK in place, pyton (via ithon) would return error message about missing .so files, which made no sense, and were impossible to figure out...