Add a requirements_dev.txt file

[mpvanderschelling]

Problem

It would be nice to add a requirements_dev.txt file that bundles the requirements for the package, testing, documentation building, flake8 check and package building.

Possible solution

Add a requirements_dev.txt file in the root folder:

# Core package requirements
-r ./requirements.txt

# Linting
flake8

# Testing
-r ./tests/requirements.txt

# Documentation
-r ./docs/requirements.txt

# Building
setuptools>=43.0.0
build
wheel

Note: this is not required for the BRG coding guidelines, just a suggestion :)