src/Classic/Algebra/ComplexEigen.cpp: use ssize_t instead of int in int ComplexEigen::hqr2() to avoid overflow

- warnings with gcc/7.3.0 fixed

- FindHDF5.cmake replaced by module shipped with CMake (see #216)
- FindGSL.cmake reviwed and adapted to the version shipped with CMake (which has some issues in newer version)
- master CMakeList.txt reviewed

- change all 'unsigned int' to 'site_t'
- length of sequence is 'size_t' not ptr to 'int'

avoid deadlock in DataSink for ParallelCyclotron if number of particles on a single core is less than 3

fixing #162; was introduced when both SBend and RBend were rewritten to be derived from 'Bend' class