Allow different optimizations at different iterations

[jdrusso]

If I pass an extra n_iter argument to my custom_optimization functions that the optimization plugin uses, I can allow using different optimization strategies at different iterations

I.e., instead of

def optimize_allocation(hamsm):

    original_allocation = westpa.rc.get_we_driver().bin_target_counts

    return original_allocation

we could have

def optimize_allocation(hamsm, n_iter):

    if n_iter < 1000:
        allocation = westpa.rc.get_we_driver().bin_target_counts

    elif n_iter < 7000:
        allocation = do_something_else()

    return allocation