fix typos -- density distributions are in rho, not u
Merge pull request #78 from numerical-mooc/convection_typos

fix typos -- density distributions are in rho, not u
fix error in LW derivation (sign flip) and change u -> rho in density profile
Merge pull request #79 from numerical-mooc/convection_typos

fix error in LW derivation (sign flip) and change u -> rho in density pr...
Merge remote-tracking branch 'upstream/master'

Update from upstream