[dune-functions] ISTL-Backend

[Carsten Gräser]

Dear all,
in my attempt to verify if out global basis interface is rich
enough I implemented an ISTL-backend whose sole functionality
is currently to provide

  operator()(const MultiIndex& row, const MultiIndex& col)

Sound's easy, but is in fact a little tricky to get right
for multitype-matrices and especially if the multi-indices
have non-uniform length (but it works for these cases).
Next thing to add would be resize/allocate functionality.
But I'm undecided if the backend should be in dune-functions
at all:

+ Stokes-example gets more elegant
+ Stokes-example gets almost agnostic on index scheme
+ Can serve as an example of how to implement a backend
- Far beyond the scope of dune-functions.
- Stokes-example gets less basic because we'd not like
  to explain the magic in the backend to Joe Average

Please give you're opinion. A 'no' would be no problem
and just mean that I put this in dune-fufem where we'll
need this once we switch to dune-functions bases.

Best,
Carsten

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