Add section on software requirements/assumptions

Add section to installation page on software requirements and
assumptions about Python program names.

Update CHANGELOG.
Update documentation on re-ordering algorithm
Further updates to build documentation
Correct command-line argument in documentation
Remove ``--past`` argument handling

Remove ``--past`` as an argument to the solve procedure in the model
template. Add ``--solve-from`` as an argument in its place.

In contrast to ``--past``, which set the period that precedes the
solution period, ``--solve-from`` sets the first period for
solution *within* the main solution span.

Update documentation accordingly.