Replace boost filesystem, any and optional if available in standard library

Here is the next set of changes for #48 (closed). They are minor API changes, no worse than what !44 (merged) introduced a few months ago (which, by the way, wasn't even tagged [API]...). This is easily fixable with something like

find . -type f -exec sed -i \
  -e 's/boost::filesystem/walberla::filesystem/g' -e 's/boost::any/walberla::any/g' \
  -e 's/boost::optional/walberla::optional/g' {} +

The nice thing about this pull request is: when building without the Python interface, we now no longer depend on any boost libraries; we only need the headers. GCC's libstdc++ ships with the necessary stuff in std::experimental since version 5.0, Clang's libc++ does too since 4.0 and MSVC since 2015. Apple's Clang doesn't ship filesystem and marks any and optional as unavailable, so we fall back to Boost on it (and on other older compiler versions).