Treating |x|<1 as a special case in floor(HighsCDouble x) and ceil(HighsCDouble x)

[jajhall]

This will close #2041

[fwesselm]

I was wondering whether it would be worth implementing ceil using floor as follows:

friend HighsCDouble ceil(const HighsCDouble& x) { return -floor(-x); }

[jajhall]

Ceil and floor are only a few lines of code, so it's more transparent to have them separate