Bug fixes and new features

Bug fixes and new features, as listed in the changes.txt file (summarized here).

Updates in v0.7.3
NOISE:

1. Added mpi option and fixed error in handling wrong file, in extract_corrdata()
2. Fixed error handling in get_stationpairs().
3. Added split_sides() to split corrfile sides and save separately.
4. Added shaping_corrdata() to wrap shaping function, convolving with wavelet.
5. Added split option in merge_pairs().

TYPES:

1. Added CorrData.shaping() to shape the data with wavelet.
2. Added CorrData.save() to wrap saving functions.
3. In CorrData.to_asdf(), save stack_method.
4. Fixed a bug in CorrData.to_egf() where the zero lag was not handled correctly. The negative side was wrong.

UTILS:

1. Added gaussian() and ricker() as the shaping wavelets.
2. Added box_smooth().

DOWNLOADERS:

1. In read_data(), changed default to False for getstainv.

HELPERS:

1. Added wavelet_labels().

MONITORING:

1. Added vpcluster_evaluate_kmean() to find the optimal number of clusters.

DISPERSION:

1. Improved frequency steps in disp_waveform_bp().

STACKING:

1. Synced with stackmaster functions.

CLUSTERING:

1. Automatically determine the optimal number of clusters.