HybridSystem multiple simulations bug

[dguittet]

Issue is that running the execute function more than once causes problems due to two reasons:

1. In the function copy for var_data, the vec field is appending entries from the rhs.vec instead of replacing.
   Bug copying vec field of var_data ssc#1157

2. The existing output data slows down simulation a lot. The data should be cleared before re-simulation.

[dguittet]

Fixed in 83bd6e0

@cpaulgilman