Some fixes to VertexClustering

VertexClustering previously only worked with data of a specific floating point type (32 bit for point coordinates). Add some templates to accept either 32 bit or 64 bit floating points for point coordintes and be a bit more careful about implicit type conversions.

I also made some changes to conform better with the VTK-m coding standards. The most common changes are using 2 space indentation for all block levels, capitolizing and using camel case for all class members, and prefixing "this->" to all use of internal class members.