* bin/common_ioap.cc, bin/common_ioap.hh (read_part_file): New
function.
* bin/ltlfilt.cc, bin/ltlmix.cc, bin/ltlsynt.cc: Use it.
* doc/org/ltlfilt.org, doc/org/ltlmix.org, doc/org/ltlsynt.org:
Mention that new option, and improve the links to its description
in ltlsynt.org.
* NEWS: Mention the new option.
* tests/core/ltlfilt.test, tests/core/ltlmix.test,
tests/core/ltlsynt.test: Adjust test cases.
Fixes#400.
* spot/tl/randomltl.cc, spot/tl/randomltl.hh: Adjust to accept
a set of formula to replace the atomic propositions.
* bin/ltlmix.cc: New file.
* bin/Makefile.am: Add it.
* bin/man/ltlmix.x: New file.
* bin/man/Makefile.am: Add it.
* doc/org/ltlmix.org: New file.
* doc/Makefile.am: Add it.
* bin/man/genltl.x, bin/man/randltl.x, bin/man/spot.x, bin/spot.cc,
doc/org/arch.tex, doc/org/concepts.org, doc/org/tools.org, NEWS: Mention
ltlmix.
* tests/core/ltlmix.test: New file.
* tests/Makefile.am: Add it.