ebos: reduce the tax rate

first, it's not a good idea to go over the whole grid for each well at
the beginning of a time step, second the Jacibian matrix of the
linearization only needs to be recreated if the well completions have
changed...
fix invalid memory access

this was caused by partial relinearization in the presence of
auxiliary equations, i.e. colIt.index() may be larger than the
dofColor_ vector. Once more, thanks goes to valgrind...