Position restraints in GROMACS files

[mattwthompson]

          It would be useful to at some point produce position restraint ITPs as well, or better yet include configurable position restraints in each molecule's ITP file. This can be done with macros and the preprocessor `#IFDEF`, `#IFNDEF`, `#DEFINE` etc thingamajigs. Position restraints can then be configured in the MDP file's `define` configuration option. Martini used to produce this (and may still do), it was very convenient and very slick. I can try and dig around for an example if it's something you'd like to do!

Originally posted by @Yoshanuikabundi in #1119 (review)