refactor GRSpec.__or__
add new function get_vars from AST
add conversions from stability, response, eventuality to GR(1)

and their unit tests
warn that stability cannot be completely converted to GR(1)
add conversion of (p until q) to GR(1) and unit test
docfix: add release operator
pep8
replace "" by '' strings

for several cases " does not need to be escaped
bugfix: operator not always defined

parent class should be agnostic of children details
avoid errors due to local import out of package
accelerate transys import

by only conditional import of graphics
add syntax check for LTL fragment convertible to set BA, so GR(1)
Merge branch 'strategy' into convert_gr1

Conflicts:
	tulip/transys/labeled_graphs.py
bugfix: missing import inside function

introduced in f11bf19765b707b2e3861d819ed887b9912d763a
import matplotlib and python only locally inside functions

to reduce running time
change default animation filetype from png -> pdf
use newax in PPP.plot_prop
expose text_color of proposition labels in PPP.plot_props
expose quiver args for vector field in LtiSysDyn.plot and PwaSysDyn.plot
bugfix: add affine term to LtiSysDyn vector field when plotting