Add window RANGE support for float4, float8, numeric.

Add window RANGE support for float4, float8, numeric.

Commit 0a459cec9 left this for later, but since time's running out,
I went ahead and took care of it.  There are more data types that
somebody might someday want RANGE support for, but this is enough
to satisfy all expectations of the SQL standard, which just says that
"numeric, datetime, and interval" types should have RANGE support.