Sample a truncated multivariate normal distribution

Generates samples from a MVN within a lower and upper bound.

There exist many implementations on FEX solving the same problem, I wasn't very confident in using them as they sometimes failed to converge (using fsolve etc) or would generate warnings complaining about the covariance matrix being close to singular.
This is a simple brute-force method that records the accepted samples, adjusts the trial size and repeats until it has reached the target sample number.