Always perform linear interpolation of calibration data.

Dependency on SciPy was removed, giving us slightly faster
execution times and less burden for installing. SciPy could
be needed in the future, if cubic interpolation is sought.
Less clutter in the CalibrationCurve object scope
Remove leftover `interpolate` argument in cli wrapper
Remove unused ConfidenceInterval class.

It may come back in the future.
Big refactoring.

- CalibrationCurve and CalAge are np.ndarray objects
- calibrate() is a method of RadiocarbonDetermination
- BP is no more an attribute of CalAge and is moved to output opts,
  for the moment always True
- R is a shorthand for RadiocarbonDetermination and preferred UI
Add a function to combine n>1 radiocarbon determinations
Do not plot invisible data, significantly faster execution