Export user-specified CXXFLAGS

Flags that are given by the user when compiling BEM++ are exported in
BemppConfig.cmake.
Replace cython template with explicit overloads

Cython in debug mode does not handle templating too well. So replace some
templating with explicit overloads.
Reworked boundary operator operations to avoid throwing in C++

Somehow, it would create incorrect calls to python's free on jenkins macs. No
idea why.