new requirement - PyYaml
	modified:   requirements.txt
Adds save and load function of lattice, closes #31

	modified:   cellular_automata/cells/base.py
	modified:   cellular_automata/cells/regular.py
	modified:   cellular_automata/lattices/equiangular.py
	modified:   test/cellular_automata/cells/test_regular.py
	modified:   test/cellular_automata/lattices/test_equiangular.py