- change contiguous from a series of global functions to separate
templated traits classes:
- is_contiguous
- is_contiguous_label
- is_contiguous_scalar
The static constexpr 'value' and a constexpr conversion operator
allow use in template expressions. The change also makes it much
easier to define general traits and to inherit from them.
The is_contiguous_label and is_contiguous_scalar are special traits
for handling data of homogeneous components of the respective types.
