Improvements to NaN checking

[ahojukka5]

1. Let's enable NaN checking somewhere in an automatic way if DCMAKE_BUILD_TYPE=Debug. It's well justified as in debug mode all optimizations are anyway disabled and performance doesn't matter.
2. Let's add option to enable NaN checking also in production code using json input file. Some kind of "enable_nan_check": true.
3. In documentation, create "debugging" page or section, describing some strategies how to debug a nonworking model. At least describe the use of abortIfNaNs. Mention also that issue Validate json file when loading configuration #31 might be a good source of NaNs if input data is missing.