Use C++11 random rather than C-style rand

[alecjacobson]

Creating this pull request on behalf of @jdumas who brings up this issue here https://github.com/libigl/libigl/pull/1597 .

There are many functions that use Eigen::Random or rand or shuffle that apparently should use C++11 style random number generators. We should do this as one pass and provide a standard to put in the style guidelines that new functions can follow.