add github actions workflows

[fcakyon]

adds continious integration (on macos/linux/windows with python 3.6-8), pypi publish and conda publish workflows using github actions

adds features:

• automatically performs style check and unittests at every push/merge to master branch ([tested on my fork])
• automatically publishes pypi package when new release is published on github
• automatically publishes anaconda cloud package when new release is published on github ([tested on my fork], [check anaconda cloud url])

requirements:

• add https://pypi.org/ username to the Secrets of the Github repository as PYPI_USERNAME
• add https://pypi.org/ password to the Secrets of the Github repository as PYPI_PASSWORD
• get an Anaconda token (with read and write API access) at anaconda.org/USERNAME/settings/access and add it to the Secrets of the Github repository as ANACONDA_TOKEN

notes:

• switched to unittest from pytest to decrease the number of dependencies used during ci