Normalize equations with constant

Equations with a constant are now normalized, too. Just not any with x.
Normalize without crashing

Testing revealed that normalizing with a constant meant that we had to
calculate a genuine lowest order still, which could be 0 if nothing else
was present in a term.
Higher order testing; testing with constants

Test equations like  y'''' = y'' + y + 2, i.e. the LHS is much higher
order than the RHS. Also tests constants and things like y'' = y' + 1,
so non-normalized (see diffeq.rb) equations.