[cmake] Make sure downstream modules find MPI if enabled

See merge request !257

Remove the files ugio.cc and ugio.h

See merge request !256

The implemented saving and loading entire grid hierarchies to files.
However, they never worked as part of Dune.  In principle we want
this functionality in Dune, but I am convinced that we should write
it from scratch, and not resurrect the code in ugio.cc.

Add include directories to library target

See merge request !255

Add architecture.h to install list

See merge request !254

evm.h: Include architecutre.h to make it more self-contained

See merge request !253

SMALL_D and SMALL_C were missing

Replace use of <float.h> by more modern <limits>

See merge request !252

Replace define by typed constexpr variable

Allocate the right amount of memory for 'node' objects

See merge request !251

These objects used to have a member of type VECTOR*, but whether
memory was actually allocated for that member was determined at run-time
(via the 'format', which doesn't exist anymore).  The VECTOR* member
was removed a while ago, but the run-time size computation was left
untouched.  This lead to valgrind complaints when load balancing.
This commit fixes the size computations.

Simplify domain creation

See merge request !250

It had been stripped to a single call of 'new', and therefore
it doesn't warrant being its own method anymore.

Because that is what it is.

This subdirectory is not used anymore.

Just treat them like regular objects.

It is now unused.

Rather than passing around BVPs by name.  This is one more step
towards getting rid of the environment heap.

After so much code has been removed, it is really just
a single assignment, which doesn't warrant a dedicated method.

'void' is part of the old abstract domain interface, which only creates
unnecessary complexity for us.

It contains the class std_BoundaryValueProblem, which I want to
manipulate directly from dune-grid.

Originally, UG3 had an abstract interface to computational domains.
However, Dune has always only used one single implementation of this
interface, namely std_BoundaryValueProblem.  All other implementations
are long gone, and I see no reason to bring them back.

As a first step towards removing the abstract interface, this commit
exposes the std_BoundaryValueProblem implementation class in a header
that dune-grid actually sees.

We had stripped everything from it but the name string.

So let's replace the entire class by a std::string.

Improve support for MSVC

See merge request !249

* Use standard file permissions
* Use pi from dune-common

It is never used.

It is never used.

It is never used.

Simplify domain creation

See merge request !248

This clarifies the ownership.

And note that in the long run, the method should be removed completely.

There is no need for that.  Simply passing them around by pointer
is much easier.

Previously, it was a static method and the BVP_Descriptor object
held a pointer to that method.  In the old days, this was
used to make the Configure method, well, configurable.

Now we just expose the method in std_domain.h and
let the UGGrid implementation in dune-grid call the method
directly.