Add warning in API concerning properties containing model variables

[AlexBork]

The parseProperties function taking only a string as input is not able to parse properties containing model variables as these are not registered with the created ExpressionManager.
This is expected behaviour, however, the resulting error message of the form

ERROR (SpiritErrorHandler.h:27): Parsing error at 1:2:  expecting <basic path formula>, here:
	X (var = 0)]
	 ^

does not point to the actual problem, making debugging the issue difficult, in particular for inexperienced users.
This PR adds a warning to the function that makes the issue more apparent.

Thanks to @SvStein for bringing this to my attention.

[AlexBork]

If you think the warning should only come up when in DEBUG mode, please let me know.

I also considered making the parser error message more meaningful, though that seems to require a lot more changes for a use cases I don't expect to come up too often.

[volkm]

The current version would always output a warning. I imagine this could be quite confusing for users who input a string without model parameters, because they have to ignore the warning.
Would it be possible to provide the warning/error somewhere in the Spirit parser such that the output only occurs when really needed? (From your comment I guess that this would be too work though?

[sjunges]

Isnt that exactly iff the error is thrown?

[AlexBork]

The error can also occur when there is a different issue with the input, so it's not an iff.
I'd like to give a hint if parsing fails in the specific context of the API call as someone who is not fully aware of how the parser works (i.e. someone like me) would assume that giving a syntactically valid formula should parse.
So the error message/warning/hint should not come up in all contexts as it might also be confusing.

Would adding an error handler to the API fucntion be an option? That way, we can give the additional hint only if the specific error occurs in the context of the API call.

[volkm]

Does the Spirit parser throw an exception which we could catch in the API call and then output the additional information?

[volkm]

The updated version looks good to me. Thanks for improving the usability.

[sjunges]

@linusheck this was what we discussed last week!