Consider adding as_symbol() method or similar

[arcondello]

Similar to numpy.asarray. This would basically be

def as_symbol(model, symbol):
    if isinstance(symbol, Symbol):
        return symbol
    return model.constant(np.asarray(symbol, dtype=double))

or something like that. We could then use it in a bunch of function signatures. Because right now it's clunky to have to do model.constant(...) absolutely everywhere.