Fix for #50. Add constants for fast data.
Added the following constants
NDS::request_period::FAST_STRIDE - denote sub-second buffers if possible NDS::request_period::AUTO_STRIDE - auto detect buffer size
If the user selects fast data and it is not supported (ie nds2) the stride automatically downgrades to AUTO_STRIDE.
Make the stride its own type.
Added these constants to the connection object in SWIG.
Added a StopNano to buffer + a helper samples_to_trailing_nanoseconds to help calculate the end time of a buffer that ends between integer seconds.