Karcher mean and draft proposal of refactoring

[astamm]

• Improved documentation of calc_shape_dist() and curve_dist();
• Implemented unit tests for calc_shape_dist() and curve_dist();
• Implemented meta-function karcher_mean() which should cover many feature spaces;
• Implemented a proposition for a refactoring of the package towards computer representations of curves and warpings as R functions instead of vectors and arrays; currently covers only open curves and allows to perform pairwise distance and distance matrix; test suite is implemented as well to track current numerical precision for maybe later improvements.

The refactoring currently adds new functions to the package and as such does not impact current use of the package.

[astamm]

If accepted, it should resolve #44 and #46.

[astamm]

Done!