Add sequential models for black oil.

This commit adds sequential solvers, including a simulator variant
using them (flow_sequential.cpp) with an integration test (running
SPE1, same as for fully implicit).

The sequential code is capable of running several (but not all) test
cases without tuning or special parameters, but reducing ds_max a bit
(from default 0.2 to say 0.1) helps with transport solver
convergence. The Norne model runs fine (esp. with a little tuning). A
parameter iterate_to_fully_implicit (defaults to false) is available,
when set the simulator will iterate with alternating pressure and
transport solves towards the fully implicit solution. Although that
takes a lot extra time it serves as a correctness check.

Performance is not competitive with fully implicit at this point:
essentially both the pressure and transport models inherit the fully
implicit model and do a lot of double (or triple) work. The point has
been to establish a proof of concept and baseline for further
experiments, without disturbing the base model too much (or at all, if
possible).

Changes to existing code has been minimized by merging most such
changes as smaller PRs already, the only remaining such change is to
NewtonIterationBlackoilInterleaved. Admittedly, that code (to solve
the pressure system with AMG) is not ideal because it duplicates
similar code in CPRPreconditioner.hpp and is not parallel. I propose
to address this later by refactoring the "solve elliptic system" code
from CPRPreconditioner into a separate class that can be used also
from here

CMakeLists.txt

@@ -146,5 +146,6 @@ if (HAVE_OPM_DATA)
 
     set_tests_properties(compare_restart_files PROPERTIES DEPENDS flow_SPE1CASE2_restart) # Compares the restart files from tests flow_SPE1CASE2_restart and flow_SPE1CASE2
 
+    add_test( NAME flow_sequential_SPE1 COMMAND flow_sequential ${OPM_DATA_ROOT}/spe1/SPE1CASE1.DATA )
 
 endif()