Make tag an unavailability shortcut

[drvinceknight]

• Remove tag constraint.
• Redefine the tags constraint.
• Update documentation.

Closes #101

[drvinceknight]

0.26s$ coverage report -m
Name                                                         Stmts   Miss  Cover   Missing
------------------------------------------------------------------------------------------
src/conference_scheduler/__init__.py                             0      0   100%
src/conference_scheduler/converter.py                           17      0   100%
src/conference_scheduler/heuristics/__init__.py                  3      0   100%
src/conference_scheduler/heuristics/hill_climber.py             18      0   100%
src/conference_scheduler/heuristics/simulated_annealing.py      27      0   100%
src/conference_scheduler/heuristics/utils.py                    26      0   100%
src/conference_scheduler/lp_problem/__init__.py                  2      0   100%
src/conference_scheduler/lp_problem/constraints.py              38      0   100%
src/conference_scheduler/lp_problem/objective_functions.py      16      0   100%
src/conference_scheduler/lp_problem/utils.py                    66      0   100%
src/conference_scheduler/resources.py                           64      0   100%
src/conference_scheduler/scheduler.py                           54      0   100%
src/conference_scheduler/validator.py                           25      0   100%
------------------------------------------------------------------------------------------
TOTAL                                                          356      0   100%