v0.10.0: Add logical operators (#71)

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

Co-authored-by: Yaman Güçlü <yaman.guclu@gmail.com>