[Processing][Feature] Add Concave Hull algorithm

This pull request adds new algorithm to Processing Toolbox - Concave Hull (AKA Alpha Shapes). There are three options that user can set:
- threshold - value between 0 and 1 that controls level of details, where 1 is equal to convex hull,
- holes - if set to False (No) inner rings will be deleted,
- multipart geometries - if set to false (and output geometry is multipart) geometry will be splitted into single parts.
It also resolves #6603.

Regards
Piotr Pociask