Cannot resize a jams::MultiArray with a std::array

[drjbarker]

jams::MultiArray::resize takes variadic arguments with no type checking so std::array is matched with this version and fails at the static assertion before getting to the std::array polymorphic version.

We should use type traits and enable_if to check if the args in the variadic version are integral. If we move up to C++14 compat then we can use std::integer_sequence (see https://stackoverflow.com/questions/24687026/implementing-variadic-type-traits).