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 <[email protected]>

.travis.yml

@@ -1,16 +1,13 @@
-sudo: required
+os: linux
+dist: xenial
 language: python
 
 # which python versions to test
 python:
-  - "3.5"
-  - "3.6"
-
-# Enable 3.7 without globally enabling 'dist: xenial' for other build jobs
-matrix:
-  include:
-    - python: 3.7
-      dist: xenial
+  - 3.5
+  - 3.6
+  - 3.7
+  - 3.8
 
 # Cache directory $HOME/.cache/pip
 cache: pip

sympde/calculus/__init__.py

@@ -1,2 +1,3 @@
-from .core   import *
-from .errors import *
+from .core     import *
+from .errors   import *
+from .matrices import *