i PI v2.0 Features list

A tentative list of features that may go into v2.0. Let's also use it to add other possible features, and implementation bottlenecks.

• replica exchange (already implemented in a rough way for temperature exchange)
• open paths
• biassing for free energy sampling -- perhaps using plumed
• RPMD - related features: constrained dynamics and calculation of recrossing
• constant stress simulations
• abstracting the communication between i-PI and the client, and providing a library that can be linked with any code and supports multiple communication modes (e.g. sockets, pipes, filesystem, perhaps inter-process MPI...)
• more general initialization / extra parameters communication protocol to return richer information and to provide more information to the client (e.g. if the replica index has changed)
• additional pimd technology e.g. NHC or staging (mostly for reference, I really do not think there is a real need, but it would be nice if someone cares)
• high-order trotter splittings
• alchemical transformation steps to simulate isotope mixtures
• particle exchange monte carlo moves?