Pycsp3

New forms are possible for posting extension constraints, regular and lex constraints. Arbitrary constraints
are now possible with adhoc constraints.

Minor changes

PyCSP3 2.2, with new (control) structures 'If' and 'Match', new derivated constraint forms (Hamming, Exist, NotExist, ExactlyOne, AtLeastOne, AtMostOne and AllHold), new functions 'both' and 'either', auto-adjustment of array indexing, and the predefined named tuple 'Task'. See other extensions in Changelog Chapter of the complete [guide](https://arxiv.org/abs/2009.00326).

PyCSP3 2.1, with new versions of embedded solvers (ACE and Choco) and additional constraints (Precedence, BinPacking, Knapsack, MinimumArg, MaximumArg). See other extensions in Changelog Chapter of the complete [guide](https://arxiv.org/abs/2009.00326).

New functions allow us to pilot the solving process.
Everything you need to know about the interface of the library is described in a new chapter.
How to format data in filenames, to use default data and to load independent JSON data files (possibly from URLs) is explained in the guide.