Refactor code base into plan-based implementation

Changes:

• Complete refactoring of the code base according to the new design posted on the wiki (fixes issue #19 (closed))
• Switch to xtensor implementation, removal of the casacore DuoMatrix implementation (fixes issue #22 (closed)).
• Preparations to start performance engineering work - better data mapping of the data components used in hotspots.

In summary, this MR fixes issues #19 (closed), #22 (closed) and #25 (closed).