Define variables through new @defvar macro

added 10 commits

• 9df59150 - Update get_* docs
• 1027e0ef - Add variables. in docs
• a8078c7b - Document variables and @defvar
• 716c1d25 - Turn variables into a module
• 882dd135 - Only access variables through variables.
• d26c9e97 - Remove duplicate variables.
• c86d3118 - Fix tests
• d72c03b3 - Document all the variables
• 7d116577 - Add variables. in documentation links
• c67b710d - Add missing docstrings

Compare with previous version

mentioned in commit 1ea539f8

merged