[Ipopt] (yet another) release 3.5.1

[Andreas Waechter]

Hi,

I was made aware today that there is a new MUMPS release (4.8.1) which 
seems to have a few bug fixes, and might be faster(...?).  I created a new 
Ipopt release that uses this new version.

IMPORTANT:

If you update your stable/3.5 branch or the trunk version now, you need to 
rerun the "get.Mumps" script in the ThirdParty/Mumps directory to get the 
latest version.  Otherwise, you will obtain compilation errors.

Regards,

Andreas