Improved usability of HistogramArray

HistogramArray could not be used for large number of histograms as compilation time was becoming too long. This was due to the usage of std::array at the back and template metaprograming to initialize it properly.

The new implementation keeps std::array for N < 30 but switched to boost's static_vector for larger arrays, leading to faster compilation and only small penalty at initialization in case N >= 30

Fixes lhcb/Lbcom#12 (closed)