Add logical operators

[saidctb]

Implementation of logical operators:

• Add 1d, 2d, 3d logical operators
• Add Piola transformations
• Allow application of LogicalExpr on vectorized operators like grad, div, curl, dot, ...
• Add MatrixSymbolicExpr class and Symbolic jacobian Matrix
• Update TerminalExpr to handle physical and logical coordinates
• Add mappig attribute to Domain class which is set to None if it's a logical domain
• Domain.coordinates can have two sets of coordinates x1, x2, x3 if it's a logical domain or x, y , z if it's a physical domain

Moreover:

• Sort Union args by their string representation (fixes recurring failure in unit test)
• Update .travis.yml (don't use "sudo", also test on Python-3.8; always use Linux Xenial 16.04)
• Update version.py