Update code style in dynamic core 2

[gassmoeller]

Follow up to #6177, this PR does the following:

• Address the comments in Update code style in dynamic core plugin #6177 (the variable n that was a double before was meant to be and also treated like an unsigned int, I checked the original reference)
• Reorder functions in our usual structure (constructor -> initialize/update -> everything else -> declare_parameters -> parse_parameters)
• Added lots of spaces around equal signs and some operators to improve readability

I think I will make at least one additional follow-up, but I will separate the PRs, because this PR should be purely code structure changes while the next one may actually affect results

[gassmoeller]

I guess this shows the dangers of relying on (or forgetting about) automatic type conversions. I fixed the accidental integer conversion in fun_Sn, test results should now be fine.