Bugfix, again: it was correct to take the maximum, reinstating.

There is a different problem that needs fixing, however:
Flux inaccuracies (for example on boundaries) may tag some
face as inflow face for a cell, even if it should have been
no-flow. This may let the cell avoid limiting, even though it
should have been limited according to the proper inflow faces.
Simplify and correct implementation of limiter.

Now we check all corners' tof values for the cell under consideration,
not just the inflow face corners'.
Add methods sequence() and components().
Improvements to DG1 flux limiter.

Skeleton in place for increased flexibility in methods and usage.
(So far behaviour choices are hardcoded, though.)
Added relative flux thresholding to existing limiter to avoid flux noise
strongly affecting solution. For example no-flow boundaries could be treated
as inflow boundaries and make the minimum upwind face limiter meaningless.
Change constructor to take ParameterGroup argument.

Also make tof to limit against >= 0.0, for case when upstream cell values go
below zero.

Disabled some debug output.
Documented new available limiting options in constructor doc.