Make machine learner section at runtime.

By default we use the mageec_ml learner unless MAGEEC_MODE is set
to FILEML. This replaces the previous preprocessor method of
choosing a machine learner.

	* include/mageec/mageec.h (ml): Replace with pointer to machine
	learner.
	* mageec/mageec.cpp (mageec_framework::init): Create either normal
	or file machine learner based on MAGEEC_MODE environment variable.
	Replace ml member references with pointer equivalent.
	(mageec_framework::new_file): Add assertion to check MAGEEC is
	initialized.
	Replace ml member references with pointer equivalent.
	(mageec_framework::end_file): Likewise.
	(mageec_framework::finish): Likewise.
	(mageec_framework::all_passes): Likewise.
	(mageec_framework::take_features): Likewise.