CI for coding-styles

[volkm]

Split from #1039

Adds a Python script resources/scripts/check_style.py which allows to check our coding style conventions described in doc/developrs.md. The current version checks:

• include-order: Order of includes is correct:
  - own header first (bare filename; test files use the "storm-config.h" + "test/storm_gtest.h" pair instead),
  - then external (<...>) includes
  - then additional ("...") includes with full path.
• pragma-once: headers must start with #pragma once.
• raw-throw: exceptions must be thrown via STORM_LOG_THROW, not a raw throw.
• doxygen-style: doc comments must use /*!, not /**.
• stdout-usage: std::cout must not be used in the library code.
  Exceptions are defined in check-style-ignore

Currently, the checks doxygen-style and stdout-usage yield too many errors and are disabled in the CI.
The include-order check is enabled but needs fixing in a separate PR.