SteklovPoincareStep class.

That way it can access all its data.  I don't know if that is better
design-wise, but for the time being it make things simpler.

[[Imported from SVN: r6845]]

The entire primal (i.e., Dirichlet-to-Neumann) should now run for
arbitrary numbers of rods and continua.

[[Imported from SVN: r6844]]

[[Imported from SVN: r6843]]

[[Imported from SVN: r6842]]

[[Imported from SVN: r6841]]

Because that is what happens.  The previous solution of returning
to FieldVectors, one as the return value and the other one using
call-by-reference, was extremely ugly.

This is an API change.

[[Imported from SVN: r6840]]

[[Imported from SVN: r6838]]

[[Imported from SVN: r6837]]

[[Imported from SVN: r6835]]

Create rod by interpolating between two endpoints, which are expected to be already in the result container

[[Imported from SVN: r6834]]

[[Imported from SVN: r6833]]

[[Imported from SVN: r6832]]

[[Imported from SVN: r6831]]

[[Imported from SVN: r6830]]

It can (almost) handle couplings to an arbitrary number of rods now.

[[Imported from SVN: r6829]]

[[Imported from SVN: r6828]]

[[Imported from SVN: r6827]]

[[Imported from SVN: r6826]]

... to n continua and 1 rod.

[[Imported from SVN: r6825]]

In a first step, the data structures are generalized.
All stuff is again kept in std::maps index by strings.
This initial patch contains a few map lookups in very
undesirable places.  This will be fixed in the very
near future.

[[Imported from SVN: r6824]]

[[Imported from SVN: r6823]]

[[Imported from SVN: r6822]]

[[Imported from SVN: r6821]]

[[Imported from SVN: r6819]]

[[Imported from SVN: r6817]]

[[Imported from SVN: r6816]]

[[Imported from SVN: r6815]]

[[Imported from SVN: r6814]]

[[Imported from SVN: r6810]]

[[Imported from SVN: r6809]]

[[Imported from SVN: r6808]]

[[Imported from SVN: r6807]]

bugfix: use operator[] instead of find() if you want to force creation of a non-existant entry in a std::map

[[Imported from SVN: r6802]]

[[Imported from SVN: r6801]]

[[Imported from SVN: r6800]]

[[Imported from SVN: r6799]]

[[Imported from SVN: r6798]]

don't store the continuum interface boundary in the Steklov-Poincare step -- use the one from the complex instead

[[Imported from SVN: r6797]]

[[Imported from SVN: r6795]]

[[Imported from SVN: r6794]]