Documentation of matrix slicing
More slicing functions for two dimensional arrays

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.
More slicing assignment functions for two dimensional arrays

Now we have the full grid of possibilities implemented: For 'M', 'N'
two instances of a domain satisfying 'TwoDimensionalArrayCategory' we
can write the following assignment to a subset of 'M'

  M(u, v) := N

where both selectors u and v can be any of:

- A single Integer
- An Integer Segment
- List of Integers
- A List of Integer Segemnts
Slicers for selecting 'all rows' and 'all columns'
Handle empty segments in 'check_seg'

Before this change, the second and fourth example failed
while the first and third did not. I think ignoring but
not failing on empty segments is the correct fix.

m([1..2,2..1], 1)

         +1+
   (90)  | |
         +4+
                                                        Type: Matrix(Integer)
m([1..3,3..1], 1)

         +1+
         | |
   (91)  |4|
         | |
         +7+
                                                        Type: Matrix(Integer)
m(1, [1..2,2..1])

   (92)  [1  2]
                                                        Type: Matrix(Integer)
m(1, [1..3,3..1])

   (93)  [1  2  3]
                                                        Type: Matrix(Integer)
Always check for empty segments if segments are in the input