Add a seed parameter and default value to random_vector function

All the invocations of random_vector still use the same seed, in
order to preserve the output of the program. This is important to
not interfere with testing. People that wish to experiment with
the example program or use the random_vector() function in their
own codes can supply their own seeds.

Fixes #390
Merge pull request #531 from jaredhoberock/issue-390

Add a seed parameter and default value to random_vector function