Update interface to support caching

Allow proxies to support caching policies by providing init_cache and flush_cache methods, to initialize and finalize a series of similar gridding or degridding calls. The gridding and degridding methods now have fewer arguments (subgrid_size, cell_size, w_step and shift are gone). This information is now stored in the Plan instead. When init_cache(...) is called information on the upcoming series of gridding or degridding calls is stored in m_cache_state. The make_plan() method with fewer arguments can then be used. The missing arguments are copied from m_cache_state into the Plan.

Currently CPU::Optimized and Hybrid::GenericOptimized use this scheme to implement w-tiling.