Update README.md
SimpleTag::units convert argument from ref to const ref

Do this so we can bind to rvalues and thus enable calls to units
with initialiser list, i.e. SimpleTag::units({"mV", "cm", "m^2"});
Merge pull request #254 from gicmo/st_unit_const

SimpleTag::units convert argument from ref to const ref
SectionHDF5: child section have now a parent

There was a bug in SectionHDF5::getSection where the parent section was not set for the returned child. This commit fixes this problem.