Add function to merge two matrices/data.frames, where one goes from time 0 to...

Add function to merge two matrices/data.frames, where one goes from time 0 to t-1, and the other from time t to the end in the resulting matrix/data.frame.

This is used later on when contract changes are done, because everything up to t-1 is from the old values, everything start at t is using the new contract data.