Make append slice templated, work for all types
Make append slice templated, work for all types

This just generalizes append slice to work for all supported data types,
tested with unsigned char and int using the test data available.

Signed-off-by: Marcus D. Hanwell <cf7042e2e8eee958b5bcde1ae2cbefef82efc184@kitware.com>

OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz