tauUpdate: Update tau
In ppsbm: Clustering in Longitudinal Networks

One update of τ by the fixed point equation

1	tauUpdate(tau, pi, mstep, data, directed, sparse, method, rho)

`tau`	Old τ
`pi`	Estimator of group probabilities π
`mstep`	Results of the previous mstep for iterative computation
`data`	Data same of mainVEM
`directed`	Boolean for directed (TRUE) or undirected (FALSE) case
`sparse`	Boolean for sparse (TRUE) or not sparse (FALSE) case
`method`	List of string. Can be "hist" for histogram method or "kernel" for kernel method
`rho`	Old ρ (only for sparse model, set to 0 otherwise)