added nbody::io namespace with two functions for reading and writing to files.  TODO: write implementation for writing
changed body data structure to hold acceleration vector. Although our basic engine doesn't use acceleration, it may be necessary for more complex engines.  In addition, the output files expect current acceleration values, so we need someplace to store them
added method to write data to files.  For doubles, it writes more decimal places than we usually need, but it's probably best for maintaining as much precision as possible