Improve performance of GUI when game has large strategic form.

* Only create the strategic form game panel when requested, rather than
  automatically when the game is created.
* If the game has a large (>= 50000 contingencies) strategic form,
  warn the user before attempting to create the view on the strategic
  game.
Update version number to 13.0.0 and verify source packaging is up-to-date.
Convert assertions to use exceptions where appropriate.

* Remove calls to assert()
* Defined a new AssertionException, to be used in situations where
  an assertion failure would ordinarily occur.
* Convert assertions to an appropriate exception, or remove
  (in some cases where rather redundant, especially in enumpoly).
Minor encapsulation improvement in enumpoly for extensive games.
Revisions and clarifications in Nash equilibrium computation dialog.

* Removed option to compute "all" equilibria for games with more than
  two players, as enumpoly is not reliable to do this in practice.
* Made liap the default for as many equilibria as possible, rather than
  enumpoly.
* Bugfix: no longer offer lp as a solution method for two-player games
  which are not constant sum.
* Added options to use Gametracer global Newton tracing and iterated
  polymatrix approximation.
Fix bug in deactivating subtrees in behavior supports.

Deactivation of subtrees in behavior supports (for support enumeration)
had a bug in deactivating at chance nodes.  This commit handles
chance nodes correctly.