Improvement to Order Selection, Error approximation in the ABM stepper

[vhartman]

• assumes constant stepsize for the next step to approximate error
• moves the complete order adjustment to the class order_adjustment
• slight changes to adaptive_adams_coefficients

[vhartman]

Closed and reopened to force a rebuild in Travis. Failed due an internal compiler error at some point.

[vhartman]

I am not sure what the problem with the build is. Maybe someone else can provide some insights here?

[mariomulansky]

Some of our unit tests require lots of memory which might fail on the restricted jenkins machines every now and then I think. I've re-triggered and it passed. I'll have a look at the PR later this week.

[mariomulansky]

Thanks!