This PR add the new restriction functions beforeTimestamp and afterTimestamp.

The same functionality can be expressed with the functions atTime, startTimestamp, and endTimestamp, for example, in the case of temporal geometric points

beforeTimestamp(tgeompoint, timestamptz) 
afterTimestamp(tgeompoint, timestamptz) 

can be expressed as follows

atTime(tgeompoint, span(startTimestamp(tgeompoint),timestamptz)) 
atTime(tgeompoint, span(timestamptz,endTimestamp(tgeompoint))) 

when

startTimestamp(tgeompoint) <= timestamptz
timestamptz <= endTimestamp(tgeompoint))

and NULL otherwise.

The new functions enable a specialized and a more efficient implementation in addition to facilitate query writing.