[Flow-v1.3] API change and improved scripts

Created by: acrovato

Context

This PR brings the latest changes in Flow to master. This version is the minimal requested to run CUPyDO-v1.0

Modifications

• Split Solver class into Newton and Picard classes, user can choose the algorithm as they see fit
• Improved polar scripts, the solver and the problem are not re-initialized at every change in angle of attack
• Created template functions in python calling the C++ classes and functions used in the tests, if the C++ code changes, then there is only one python file to adapt
• Added CODEOWNERS file
• Minor cleaning/refactoring
• Minor refactoring by @rboman

Tests

Tests are passing on debian-4.9 (gaston), ubuntu-16.04 (shippable, no Trilinos) and ubuntu-18.04 (my VM)

Comments

• Regarding the template functions I created for flow tests and validation (flow/template.py): I am not sure about this new method, I will probably refactor it into real python API in the future
• Regarding the CODEOWNERS file: this is a test, I will try to PR without adding any reviewers