Simplify extern template declaration

For C++11 (and for some pre-C++11 compilers via extensions) a template
instantiation can be declared "extern" so that the compiler will avoid
generating more than one copy of the code for that template.

This commit is an attempt to simplify the use of extern template in VTK.
1 job for extern-template2
Status Job ID Name Coverage
  External
passed kwrobot-commit-check