[Ipopt] New Ipopt release 3.9.1

[Andreas Waechter]

Dear Ipopt users,

I created a new Ipopt release (3.9.1).  The changes include the following:

- Switch to the new COIN BuildTools system.  The location of the libraries
   and header files in the install directories has slightly changed, and
   we now support pkg-config for the COIN projects.  This new setup has
   been tested for a while, but in case you run into problems, please email
   the mailing list.

- Improved quasi-Newton approximation for the restoration phase.  The
   previous versions were not working well in case the Hessian matrix was
   approximated and the algorithm switched to the restoration phase.  In
   case you are using Ipopt with Hessian approximation, these recent
   changes might improve the performance in your setting.

- Interface to the R Project for Statistical Computing.  Jelmer Ypma
   contributed interface code for the R language (see
   Ipopt/contrib/RInterface).  Thanks, Jelmer!

- Intermediate callback functionality now available from C and Fortran
   interfaces.  The C and Fortran examples show you how to use it.


Regards,

Andreas