Continuous sampling: linear, random, from simplex

Linearly and randomly sampling points in simplex.

Samples continuous points in an interval. It allows for both linear and
uniform random sampling. It is also possible to draw points only in the
simplex of the interval.
Inputs:
- lo : lower bound (column of length D)
- hi : upper bound (column of length D)
- N : number of points to sample
- useSimplex : put 1 if you want to sample from the simplex of the bound, 0 otherwise
- linearly : put 1 if you want the points to be linearly spaced, 0 otherwise
Outputs:
- points : D-by-NN matrix with random points, where NN >= N

More information about sampling from the simplex:
http://math.stackexchange.com/questions/502583/uniform-sampling-of-points-on-a-simplex