Provide C++ interface

[vkuznet]

We'd like to use c2numpy tool within CMSSW C++ framework. Other HEP experiments may have similar requirements. Therefore it would be nice to have a C++ c2numpy interface/APIs.

[jpivarski]

Will do. An RAII style constructor will be safer (can't create writer without initializing it) and the destructor will contain a call to close, so that forgetting this step doesn't create unreadable files.