- lot of using std or using xfem or using std::string etc ..  were still present in global namespace in some header. Following a discussion with A. Salzman, I removed most of them.

    - In the transition from mPoint to xPoint, Some using point = xtensor::xPoint were defined inside some class (xMapping, xElementfor example... ). They were identifyed as confusing by A. Salzman and are now removed.

    Most of the changes in this commit reflect the previous change. The guideline is to avoid hiding namespace at least at global scope in header-file. This can avoid some confusion and some surprising and hard to track effects where some stuff compil or not depending on the order of inclusion of header files.