Based on #238 and #242 I assessed the recent changes in #268 and improved the changes

• added more test cases for different manual record_durations
• due to the conversion to long long we are limited to record durations between 0.001 and 60, which should be fine for most cases
• checking for str(float) can lead to output of scientific notation, so force floating point notation with f{x:f}
• To check for valid writing of record_duration, read the raw header instead of EdfWriter, as a safety check.

fixes #238