New [g]lmerControl() arg.  'check.rankX' (with 5 different options)
instead of  ensureXrank = T/F.
Adapt checks and help page
chkRank.drop.cols(): be more consistent in use of rankMatrix();
new 'method' argument with default "qr.R", but no intention to allow the
user to change that easily.
not *requiring* to recompute summary(<merMod>), just because correlations
should be printed
resurrect  'star' data example -- only for testLevel >= 3
vcov()/SE calculation straight from Hessian

when possible (add use.hessian argument to vcov and summary)
closes #47
version bump
correct lmer objective function to fully account for the influence of prior weights
prior weights tests and commenting
prior weights documentation
update NEWS
minor NEWS edit
update d_ldW in setWeights
remove old debugging code
vcov.merMod():  as(V, "dpoMatrix")  needs strictly symmetric matrix;
	{failed in profile example's   pr4 <- profile(gm1)}
slightly less precise "expect_equal"  {and cosmetic}
numerically symmetrize before forceSymmetric(.)  maybe very slightly nicer.
comments (commit & push before Ben fixes issue #172 :-)
another comment fix
check symmetry of fixed effect variance-cov matrix
add more basic nlmer examples