More slicing functions for two dimensional arrays
Fix a logical typo
Add a comment about some slicing methods
More slicing operations: allowing lists and lists of segments
Slicing of single rows and columns

These two functions return rows and columns as instances
of % instead of Row and Col domains like the row, column
commands do.
Even more slicing functions

Now we have the full grid of possibilities implemented: An instance 'M' of
a domain satisfying 'TwoDimensionalArrayCategory' is, in the simplest case,
indexed by two selectors like:

  M(u, v)

where both u and v can be any of:

- A single Integer
- An Integer Segment
- List of Integers
- A List of Integer Segemnts

All these functions can not only slice out subsets of M but also create
new, larger versions by repeating some parts as specified by repeated
selector entries.