posteriordistribcurve: posterior mean
In growthrate: Bayesian reconstruction of growth velocity

Internal function. Finds the posterior mean on the fine grid tgrid for every subject.

1	posteriordistribcurve(muhatmatrix, Sigmahat, sigma, tobs, d, YI)

`muhatmatrix`	Posterior mean at the observation times `tobs` obtained from `posteriorobs`.
`Sigmahat`	Posterior covariance at the observation times `tobs` obtained from `posteriorobs`.
`sigma`	Infinitessimal standard deviation of the tied-down Brownian motion in the prior.
`tobs`	Row vector of n observation times (in increasing order, same for each subject).
`d`	Number of time points on the fine grid.
`YI`	An N times (n-1) matrix with rows given by the values of y_i, i=1…, n-1 for the N subjects.