use SWIG `%template` more

[casperdcl]

e.g.:

https://github.com/SyneRBI/SIRF/blob/8eb31b1e47736c6d3ea9f5de7fd17fd3f6f14c5b/src/iUtilities/iutilities.cpp#L65-L68

Now that we've dropped MATLAB, you could do this in the SWIG interface file instead ^swig_template

%include "sirf/iUtilities/DataHandle.h"
%template intDataHandle dataHandle<int>;