Fix float precision (#649)

The issue is that numbers like
2.01 or 3.01 can not be precisely represented with binary floating point
numbers.

This replaces all occurrences of 'std::numeric_limits<T>::digits10 + 1' with
'std::numeric_limits<T>::max_digits10'.

Background:
Using 'std::numeric_limits<T>::digits10 + 1' is not precise enough.
Converting a 'float' into a 'string' and back to a 'float' will not always
produce the original 'float' value. To guarantee that the 'string'
representation has sufficient precision the value
'std::numeric_limits<T>::max_digits10' has to be used.