To enable the use of standalone rational expressions described in xml
	files, add the OUTPUT_TYPE_EXP and INPUT_TYPE_EXP possibilities.
	Split output_type in output_aut_type and output_exp_type to have
	different default output for automata (xml) and rational expressions
	(exp).
	* taf-kit/src/common.hh: split output_type.
	* taf-kit/src/main.cc: split output_type.
	* taf-kit/src/pipe.hh: Add *_TYPE_EXP.
	* taf-kit/src/pipe.hxx: split output_type.
	* taf-kit/src/pipe_writers.hh: split output_type.
	* taf-kit/src/pipe_writers.hxx: split output_type. Prepare to support
	xml standalone rational expressions' files. Improve the error
	messages.