Both u and a were written to and read from the field 'u'

- The configuration parameter io.writeVTK is now io.vtk.write
- The configuration parameters restarts.first and restarts.spacing
  were moved into the io namespace
- Restarts are stored in a separate file
- VTK data, restarts (in HDF5), and the other HDF5 data are now
  each controlled through a parameter:
    io.data.write, io.restarts.write, io.vtk.write

In particular, it is thus now possible to write VTK data while
opening the data and restart files in read-only mode (or not at all);
This makes it possible to, by keeping the restarts file (and the binary!)
around, quickly (re-)compute a a small segment from the history.

The likeliest scenario: VTK files are huge and for a computation with
tens of thousands of timesteps (e.g. 30000), we know already (from the
data, or just expect it) that an event occurred during the last 500
timesteps. So we recompute them from a restart in the following mode:

  -io.restarts.first 29500
  -io.restarts.write false
  -io.data.write false
  -io.vtk.write true

Note that care needs to be taken to use a number for io.restarts.first
which is compatible with the old io.restarts.spacing

If either signal is detected, we try to terminate gracefully. If we're
given enough time before a nastier signal like SIGKILL is sent, we'll
then be able to finish computing and writing out the current timestep.

If we do not take such precautions, chances are the HDF5 output will be
corrupted.

This is now necessary even though the code is entirely serial.

This works because interpolateP0ToP1 is in dune-fufem now

Prior to this change, dune-tectonic was incompatible with
dune-fufem releases/2.4-2 and later, thus also releases/2.5-1

Prior to this change, dune-tectonic is incompatible with
dune-fufem releases/2.4-1 and earlier

This allows us to remove the last boost::filesystem dependency, and
thus all of boost. It seems to be a better idea than hardcoding the
path to the source directory (which might be unavailable later) anyway

This can be used in conjunction with paraview to interpolate data from
an unstructured, fine grid (on which plotting a vector field would
lead to a mess) onto a structured, coarser grid using the 'Resample
with DataSet' filter.