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>

v0.9.10

Update version.py (#72)

v0.9.9: Devel ddm (#67)

* Don't split the subdomains integral when calling terminalexpr

* Improve printing of domains and boundary

* Add NCubeInterior

* Calculate dtype and dim of domain

* Allow splitting over domains for the Norm

* Delete function _split_expr_over_subdomains

* Add assert statements in topology tests

* Don't override dim property in Interval

* Update version.py

v0.9.8

Update version.py

v0.9.7

[mapping] add SymbolicWeightedVolume class

v0.9.6: Devel bounds (#60)

* refactor: Use 'args' property instead of '_args' attribute.

As described in Sympy's User Guide, one should never use internal
methods of variables, prefixed with '_' (as an example, it is explicitly
written that one should not use '_args', but use 'args' instead).

Here we apply this rule to the two main modules in the 'topology'
directory: 'basic' and 'domain'.

* refactor: Improve Boundary class

* refactor: Remove obsolete module

* feat: Add coord bounds to Line/Square/Cube domains

Several changes in 'domain' module:

* It is now possible to instantiate objects of type Line, Square and
  Cube (these classes were only used as Domain constructors before);

* Line, Square and Cube are subclasses of the generic NCube class, they
  have different constructor signatures, and use NCube's constructor to
  instantiate an object of their type with minimal code duplication;

* NCube is a subclass of Domain; its constructor returns a Line object
  for dim=1, a Square for dim=2, a Cube for dim=3, and a generic NCube
  for dim>=4;

* Line, Square, Cube, and NCube allow the user to prescribe the
  coordinate bounds, which are (0, 1) by default.

Moreover, unit tests for the new features are provided.

* chore: Do not use latest version of Sympy (1.5)

* refactor: Avoid Codacy errors

v0.9.5

Cleanup and improvements in expr module after introduction of integra…