Add function to glue two SWC files together

Some tracing software / users save each branch of a neuron as a separate SWC file, which is incorrect and severely limits the functionality of nat when used with these files. This function provides a way to 'glue' these neuron branches back together into one SWC file, via a user-selection of a point from each neuron that should be joined together. This function assumes that the SWC files are completely unbranched, and will give erroneous results if used on SWC files that contain branches (unless the branch points are all lower down the tree than the selected glue point).