Adds communicator collectives.

[rogerpearce]

Starts a collective.hpp file to hold common collective operations.

Currently supports:

• sum
• min
• max
• prefix_sum
• logical_and
• logical_or

Also, improves detail::mpi_typeof()

[steiltre]

We should have a copy of the ygm::comm::all_reduce() that takes a generic merge function in collective.hpp as well.

[rogerpearce]

Just added ygm::bcast and ygm::is_same. Fixed some inlines for non templated functions.

[steiltre]

Going to merge this in to begin using. We can continue adding missing functionality to the new collectives since the old ones will be left inside ygm::comm for now.