Improvements to the grammar

[Valo13]

When contacting the developer of dparser concerning issue #1665, he did not only fix and improve dparser to fix this issue, but he also gave some advice on how he would expect the grammar of mCRL2 to be written, see this issue, including a draft version. Since the specific issue on our end has been fixed, improving the grammar right now does not have high priority, but it might be worthwhile to do in the future.