add toString() to all executable operators

[hartig]

... to make debugging output and mentioning such operators in exceptions look more informative.

[diptechexpo]

I am interested to contribute in this project. However I am not seeing any setup guideline of the project in the documentation. If there are setup instructions available, can that be shared?
I am new to open source and looking for a good-first-issue to begin.

[hartig]

@diptechexpo thanks a lot for your offer to contribute. Much appreciated!! Yet, the timing is a bit unfortunate, as we are all on vacation at the moment (we have relatively long summer breaks in Sweden). Additionally, we have just started putting an effort in documenting the project and making it easier to use the engine (and, eventually, also to contribute to its development).

Having said that, at least there is already some information for potential contributors in the wiki of the repo, including the following page with some "getting started" instructions.

https://github.com/LiUSemWeb/HeFQUIN/wiki/Getting-started-as-a-contributor

If there is anything unclear in these instructions or you think something important is missing, please create a separate issue about it.

Other than that, feel free to create a PR that addresses this issue (or ask if you have any more questions about it).