Noninformative prior s^2 was stupid

Noninformative should be a small *h*, i.e. a large s^2
Split libcreativity.so into non-GUI and GUI parts

Everything under creativity/gui now goes into libcreativitygui.so,
which only creativity-gui, not creativity-cli, depends upon: thus
creativity-cli can be used on systems without libgtkmm installed.
Warn if not using result of Linear::update
Made Linear::update public
Fixed s2_post calculation in update

The value was being calculated incorrectly (it was missing an
inverse)--but rather than calculate this inverse, switched it to just
use the calculation from JGM's Bayesian notes (from Koop), as it is
equivalent but doesn't require an (extra) inverse, and similarly to the
fixed version of the old solution, doesn't require X'X to be invertible.
Added test script for testing sequentual updating
Increased the prior_n for noninformative models

The 1e-6 was failing catastrophically if attempting to draw() from the
noninformative model because getting the t-distributed value requires
getting a chi-squared(n) value, but for 1e-6, such draws are usually
values below machine epsilon.  Even 1e-3 still produces <epsilon draws
about 2.4% of the time.  1e-1 and 1e-2 seemed okay, but just to be on
the safe side, I bumped this up to 0.5.

Hopefully that doesn't make it too "informative", but coupled with the
large noninformative s^2 value of 1e+6, I think this will still be fine.
Changed the description & to 'and'
Made initial beliefs noninformative

This simplifies the CLI, and it isn't clear that there was a particular
good default anyway: a noninformative prior (that gets quickly updated)
seems better.