Autogenerate the implementation make and explicit template instantiation

[mormj]

The following code is boilerplate and should somehow be autogenerated away:

template <class T>
typename agc<T>::sptr agc<T>::make_cpu(const block_args& args)
{
    return std::make_shared<agc_cpu<T>>(args);
}

and

template class agc<float>;
template class agc<gr_complex>;

This should all be inferred from the yml file - will probably require another compilation unit