Added additional functions

The following functions were added: 
- timePointsOfInterval: gets the start and end points of a textgrid interval
- toIntensity: creates and saves a Praat intensity object given a .wav file
- intensityMaximum: gets the maximum intensity from  a Praat intensity object given start and end points (which can be gotten using timePointsOfInterva)
- intensityMaximumTime: same as above, but instead of the value of the maximum intensity it returns the time of that value (will be used in future functions)