add 'sec' as valid unit to util::convertToSeconds
add isScalable util method to avoid try-catch in client code
add conversion of scalable si units to util::convert methods

move util::convertToKelvin and Second to allow using util methods
test for util::conversion functions
move checks from util::getSIScaling to util::isScalable
bugfix in docstring of util::convertToKelvin
Merge pull request #282 from jgrewe/conversions

Some additions to util temp and time conversion functions

LGTM