[cmake] added boost filesystem lib
[linearelliptic.discretizations.base] add copy ctors
[linearelliptic.discretizations.base] fixed ContainerBasedDefault
[examples.linearelliptic.cg]
* readded config.h include (needed for the python bindings)
* readded discretization_and_return_ptr()
[exmaples.linearelliptic.cg] updated python bindings