twa: rename the is_* getters as prop_*
This fixes #116.
* src/twa/twa.hh: Rename those methods.
* NEWS: Document the renamings.
* doc/org/hoa.org, doc/org/tut21.org, src/parseaut/parseaut.yy,
src/tests/ikwiad.cc, src/twa/twagraph.hh,
src/twaalgos/are_isomorphic.cc, src/twaalgos/complete.cc,
src/twaalgos/degen.cc, src/twaalgos/dot.cc, src/twaalgos/dtbasat.cc,
src/twaalgos/dtgbasat.cc, src/twaalgos/hoa.cc, src/twaalgos/isdet.cc,
src/twaalgos/isunamb.cc, src/twaalgos/lbtt.cc,
src/twaalgos/minimize.cc, src/twaalgos/postproc.cc,
src/twaalgos/product.cc, src/twaalgos/randomgraph.cc,
src/twaalgos/remfin.cc, src/twaalgos/sbacc.cc,
src/twaalgos/simulation.cc, src/twaalgos/stutter.cc,
src/twaalgos/totgba.cc: Adjust.