Mismatch of pattern concept and implementation

[ddiefenthaler]

In some patterns the pattern concept is only implemented very shallow. For example the ShiftTilePattern even differs in the interface for the 1-dim and n-dim variants.

Here is an incomplete list:

• index global(unit u, index li) isn't implemented for most patterns
• size[d] local_extents(unit u) isn't implemented for ShiftTilePattern
• size[d] local_extents() is allowed while not listed in the concept.

This list only includes what I have found for now.

This issue should also track the implementation status, when issue #507 is adopted.