calculate_transitionMatrix: Calculation of the transition matrix.
In tbuder/CellTrans: Quantifying stochastic cell state transitions

This function derives the transition matrix which contains the transition probabilitites between the distinct cell states.

1	calculate_transitionMatrix(M, t, used_timepoints)

`M`	Matrix containing all experimental data: the initial experimental setup matrix and the experimental cell state distribution matrices.
`t`	This A vector containing all timepoints at which the cell state distributions have been experimentally measured.
`used_timepoints`	A vector with the timepoints utilized to estimate the transition matrix.