Add checking for unconvertible functions.

Currently, there's only one uncovertible function, and that's round().

We could probably implement support for round, but it'd be more
involved, I'm guessing that no one uses it anyway. Just in case
though, the converter will throw an exception and die to tell us if
anybody used it.
Remove extra spaces from decimal numbers.

Numbers like 5.1, after conversion from MathML to Latex, seems to gain
extra spaces on either side of the dot, e.g.: 5 . 1

Just added a check for the extra spaces and removed it.