Sparse Sequence Iterators constructors with only beg/end arguments

[jgdumas]

Both iterator and indexed iterators now have constructor with only beg/end (on rows)
then subiterator (on columns) is set only if beg != end (that is if matrix is non-empty).

In the whole LinBox library, the above constructor suffices.
When beg != end it initializes to beg->begin(); but the latter was ill-defined otherwise, causing some instability.

If needed, it is simple enough to reintroduce the 3 arguments constructor,
for when it is known in advance that beg != end, and when the third argument is different from "beg->begin()" ...

[jgdumas]

Also correcting the '==' and '!=' operators for these iterators:
test subiterator only if rows are non-empty