Added MultiEcho sequence. Refactored to use a vector<shared_ptr<SequenceBase>> instead of SequenceGroup, because that now only manages SteadyState sequences.
Added MultiEcho sequence.
Refactored to use a vector<shared_ptr<SequenceBase>> instead of
SequenceGroup, because that now only manages SteadyState sequences.

spinicist/old_QUIT,spinicist/old_QUIT,spinicist/old_QUIT,spinicist/old_QUIT