Make sources cloneable

[ayllon]

This is needed so the Python wrapper can make copies of the sources if needed: for instance, if the Python code calling the C++ one is keeping hold of a source we can not move it, so we need to clone it.

Note that the properties already computed are not copied, since they are immutable we can just share a pointer to them. However, the map itself must be copied to avoid potential race conditions.

[marcschefer]

I'm thinking it could be a good idea to have a different branch for the whole SX++ from Python project so that we can keep testing independently. And only merge once we're happy about the whole thing. What do you think?

[ayllon]

I'm thinking it could be a good idea to have a different branch for the whole SX++ from Python project so that we can keep testing independently. And only merge once we're happy about the whole thing. What do you think?

Ok!