Move null Preconditioner::comm_pt check inside ifdef MPI

It was breaking some preconditioners in serial + paranoid mode (in
serial the comm_pt should be null).
Remove some unnecessary includes in preconditioner.h
Remove Interpolator class

* Only needed for one (older) implementation of implicit midpoint rule,
  so no point leaving it around.
Remove ConvergenceData class

Added by me, but I'm not using it anymore (I found it easier to
implement something equivalent in my own problem class). Removed since
it is not used by anyone else as far as I know.
Add an ODE element
Work around lack of diff -N support on Wulfling
In autogen.sh fix bug when -r was used in a fresh build

autoreconf was called before the makefile_list was generated, and so
failed because the list of makefiles to create with configure was empty.
In autogen.sh use a cleaner method to modify automake options

Avoid modifying configure.ac in autogen.sh. Instead #include another
macro file (which is generated by autogen.sh) containing the automake
init command.

This means that configure.ac is not modified by the build process, which
makes version control much easier.
Remove all *.dir_list files and associated variables

* The list of Makefiles is automatically generated by autogen.sh. So
  there is no need to have manually updated dir_list files anymore.

* Some variables in Makefiles relied on these dir list files but did not
  appear to do anything (and I checked with Matthias) so they have been
  removed. The variables are:

    USER_DRIVER_SUBDIRECTORIES
    USER_SRC_SUBDIRECTORIES
    PRIVATE_DIR
    HAVE_PRIVATE_USER_SRC
    PRIVATE_SUBDIRECTORIES
    PRIVATE_USER_DRIVER_SUBDIRECTORIES
    HAVE_PRIVATE_USER_DRIVERS

* The variable HAVE_PRIVATE_DIRECTORY was used to decide if we should
  recurse into ./private when running make clean etc. I've replaced this
  by just running the command with a `-` prepended (so that it is
  allowed to fail without stopping make).

  Unfortunately I can't test it still works in exactly the same way
  because I've never used the private folder.
Add configure option: wulfling paranoid without doc
Force git to keep the directory config/configure.ac_scripts
In autogen.sh fix bug when using current options file

* Tried to copy 'current' file to itself, which gives an error, wrap in
  an if to avoid this.
Initial commit of a working ODE example
In ode demo driver add command line args and also test IMR
Add implementation of trapezoid rule

and a very basic self test using an ODE.
Add ODE self test using the "real" implementation of IMR
Improve comments in imr code
Rename midpoint method to implicit midpoint rule

(this is what most people actually call it)
Improve comments in trapezoid_rule.h
Merge all ode tests together and refactor