Provide C++ bindings for BeamBeam4D, BeamBeam6D, etc.

[martinschwinzerl]

Currently, the be_beamfields element do not feature a C++ version. Also, the C implementation is not fully ready yet to actually create one, most notably because the beam elements are implemented as flat unstructured stores of real-numbers rather than nested structures.

In order to fix these the following steps would be requried:

• Refactor the C99 version around nested structures and value-type arrays
• Provide a proper, fully templated version of the faddeeva function
• implement a generic template version of the beam - elements
• provide specializations for scalar 64bit double and 64 bit integer types