Fix filenames: do not use .C for (essentially) templated header files

When compiling using Eclipse CDT, *.C files are treated as standard C++ source files. However, since two of them contain template code and are actually meant to be included (!) by other C++ files, Eclipse would fail compilation. To fight this, the offending files have been renamed from *.C to *-inl.h (loosely based on the Google Style Guidelines). Additionally, the provided example.C file no longer includes the (now renamed) file Seb_debug.C, but Seb_debug.h instead (which includes Seb_debug-inl.h).

I believe this way standard C++ practice is followed more closely: header files (including template code) goes into .h (or .hpp), source files go into .C (or .cpp, etc.).

This patch therefore contains the following proposed changes:

* Rename templated .C files to -inl.h (to prevent confusion);
* Complement (add missing) and reorder (alphabetize) includes;