Update the gecode interface for gecode 5.x

[jamesjer]

This is a one-way trip, unfortunately. You gain the ability to use gecode 5.x by losing the ability to use gecode 4.x. The preprocessor can deal with the code, but I don't know what to do about gecode.ampl. This resolves #109.

[fdabrandao]

Which source did you use at mp/thirdparty/gecode to in order compile? We tried with 5.0 and 5.1 and we got several errors with both.

[jamesjer]

Sorry, I overlooked the github email! I'm using 5.1.0, on an x86_64 Fedora 31 machine. What errors did you get with 5.1.0?

[mapgccv]

Thank you @jamesjer: once i update the tests, this seems to work for the current version (6.2) too.
I think that we'd simply note that only gecode 5+ is supported with the current mp repo; in the unlikely case that people want to use older versions, they'll simply have to check-out an older commit of this repo.