Replace boost dependencies

[craffael]

This PR removes the following boost libraries from LehrFEM++:

• boost::timer (a replacement timer has been implemented in lf::base)
• boost::filesystem (not needed anymore because the standard library now provides this functionality. But I had to upgrade the minimum gcc version which is required by LehrFEM++ from 7 to 8 for this.)
• boost::chrono (was never used)
• boost::system (needed by boost::filesystem and boost::timer)

The advantag of removing these dependencies is that LehrFEM++ compiles much faster in the absence of a suitable Hunter cache (e.g. the first time that someone download the library or when the cache becomes invalidated because of compiler update). The only remaining boost dependencies are now:

• boost::program_options (only needed to build examples/projects)

@hiptmair I've implemented this on my own because I always hated to wait 20mins everytime i changed something on the compiler ;) (I didn't record the hours for this change)

[hiptmair]

Good idea to reduce dependencies. Have you really replaced boost::program_options completely?

[craffael]

No, boost::program_options is the only compiled library from boost that I didn't replace ;) If it's fine for you, I will merge it into master once all unit tests works as expected?