The Adams-Bashforth-Moulton stepper has now also the initializing stepper
as a template parameter.
This allows to get rid of the specific test case for multi-step methods in
order_quadrature_formula. Furthermore, some cosmetic adjustments were made in
this test: global variables, camel case naming, while loop -> for loop.
* state_type is now a double
* rhs is global
* p is a member of rhs, called exponent
* 'main loop' is of the form
do{ exponent++; ... } while ( error < tolerance );
