pypima/pima.py: Add Pima.source_list function

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pypima/fri.py: Support obs selection with max SNR for given station
pypima/pima.py: Pima.coarse returns fri-file name now
pypima/pima.py: Pima.fine doesn't change fri-file name now.
RaExperiment._select_ref_sta -- new function for sta_ref selection