Do not skip whitespace while reading quotient

[sloriot]

Fixes issue mentioned in #7927

sentry also eats end-of-line and arrangement formatter has a skip-until-eol after reading vertex coordinates

We decide to not allow whitespaces between num / dum

[mglisse]

We decide to not allow whitespaces between num / dum

IIUC, after the patch, you accept whitespace before num, before den, but not between num and '/'.
This seems to match what Gmpq (more generally read_float_or_quotient) does, and is less strict than gmpxx (which also refuses a space after '/'), so it shouldn't break much.
Ideally there would be a bit of testing and/or documentation to go with it, but I can understand if you don't think it is worth the trouble.

[sloriot]

Actually it's doc with no space at all around /. I can force no space after if you think it makes sense.

[mglisse]

I can force no space after if you think it makes sense.

Bah, as long as it isn't causing problems, it can wait...

[sloriot]

Successfully tested in CGAL-6.0-Ic-169