Merge branch 'feature/kostyantyn/doxygen' into 'master'

Doxygen and improvements to bit manipulation library

This merge request suggests the following change:
- Doxygen documentation for C code 
   - Updated makefiles and configure script. Doxygen documentation can be built using either `doxygen` target (which builds only doxygen) or `doc` target that builds all documentation including doxygen. Documentation is generated in `doc/doxygen/html`.
   - Several files were updated to include doxygen comments. To avoid confusion with ACSL annotations `/*! \tag */` syntax was used.
- Several small updates and improvements to bit manipulation header (`e_acsl_bits.h`).

See merge request !36