Fix grammar by adding new tokens to support not triple arguments add new checks

[likeamahoney]

According to the IEEE Standard for Standard Delay Format (SDF) 1497-2001, there are many instances in the grammar where a single float or a float enclosed in parentheses can be used instead of a float triple. I have extended the grammar to include these options.

I have also added timing checks for PERIOD, SKEW, and BIDIRECTSKEW.