This uses a concept similar to what std::valarray and std::slice do.
A labelRange provides a convenient container for holding start/size
and lends itself to addressing 'sliced' views of lists.
For safety, the operations and constructors restricts the given input range
to a valid addressible region of the underlying list, while the labelRange
itself precludes negative sizes.
The SubList version is useful for patches or other things that have a
SubList as its parameter. Otherwise the UList [] operator will be the
more natural solution. The slices can be done with a labelRange, or
a {start,size} pair.
Examples,
labelList list1 = identity(20);
list1[labelRange(18,10)] = -1;
list1[{-20,25}] = -2;
list1[{1000,5}] = -3;
const labelList list2 = identity(20);
list2[{5,10}] = -3; // ERROR: cannot assign to const!
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