Some changes to the Vec class and VecTraits in anticipation of creating
Vec-like objects. The following changes are made:
* Add GetNumberOfComponents to Vec, which returns NUM_COMPONENTS.
* Likewise, all VecTraits have a GetNumberOfComponents method.
* The ToVec method in VecTraits is changed to CopyInto so that it can be
used when the length of the Vec-like is not known. CopyInto is also
added to Vec.
* VecTraits has a typedef named IsSizeStatic which is set to
VecTraitsTagSizeStatic when the number of components is known at compile
time and VecTraitsTagSizeVariable when the number of components is not
known until runtime.
Providing these types tends to "lock in" the precision of the algorithms
used in VTK-m. Since we are using templating anyway, our templates
should be generic enough to handle difference precision in the data.
Usually the appropriate type can be determined by the data provided. In
the case where there is no hint on the precision of data to use (for
example, in the code that provides coordinates for uniform data), there
is a vtkm::FloatDefault.
