Support for different container types in saltatlas::dhnsw::dhnsw::partition_data()

[bwpriest]

In order to achieve type safety, we templated saltatlas::dhnsw::dhnsw::partition_data() on a map-like type signature Container<index_t, point_t>, and all of the lambdas that saltatlas sends to these containers expect to iterate over std::pair<index_t, point_t>. This works perfectly well for ygm::container::map, as was originally intended. However, it requires the awkward workaround ygm::container::pair_bag to work with distributed bags of key-value types, and does not work at all with other containers of interest. For example ygm::container::array does not work both because the template parameter orders are switched, and because the lambdas expect to iterate over (const index_t, point_t).

In order to provide a more general data interface to saltatlas, we can either modify ygm to make the containers accept a uniform lambda format, or maintain different partitioning implementations for saltatlas that can work with the different data structures.