CMake: robust version extraction for P4Est

Make the version extraction in P4Est more robust: We should not rely on
the version string to define SUBMINOR and PATCH - this doesn't have to
be the case. So, always set P4EST_VERSION_SUBMINOR and
P4EST_VERSION_PATCH to "0" in FindP4EST.cmake if no valid number could
be extracted.
Introduce the concept of containers-of-cells and link to it in various places.
Add some description of when the last two IFs can happen in version
number recognition.
Merge pull request #17 from bangerth/fix_p4est_configuration

Fix p4est configuration
Merge pull request #13 from bangerth/doc-fixes

The previous version of the documentation basically stated why the function exists, not what it does. This is the focus of the new version. I also indented the arguments of this function and the (related) following one.
Merge pull request #15 from bangerth/doc-fixes-2

Introduce the concept of containers-of-cells and link to it in various places.
Fix doxygen formatting.

Some of the member functions in smartpointer.h did not have documentation according to a format that doxygen would pick up.