Consistently use "" or <> in includes

[volkm]

Following this dicsussion on whether to use #include ".." or #include <..>.

If I read the documentation correctly, the quotation marks are only relevant if we want to include header files relative to the current files. This is the case for all stormpy header files. But storm is an external dependency and the files are not relative to the current file. So I would prefer to use angle brackets, but I might be wrong.

Regardless, we should agree on one standard and use it consistently.

[sjunges]

As far as I understand, <> signals that we use system libraries. Storm, when being fetched, is not in a system path.

[volkm]

I see your point, but shouldn't libraries such as carl also be included via "" then? They currently also use <>.

I personally would prefer to keep using <> to indicate that Storm is a dependency and not part of the repository.

But as long as we are using one version consistently, I am fine.

[sjunges]

I am fine with #include <..> as it seems to work. I guess we were inconsistent before and I changed some of them as I thought it may be part of a bug that I was hunting down.