remove merge, index; retrieve gaps, overlap; plot gap;overlap with different color

closes #68 (closed), #67 (closed), #57 (closed)

• Read header to check for available data; start all data dict for the channel so that if even no data, the requested channels if available will still be plotted with empty lines; add header to the channel's stream to retrieve gap/overlap after all data are read.
• Read data of a file right after the file's trace headers are checked so the process of indexing and reading from index will be removed.
• All data (soh, mass pos, waveform) are treated in the same way, only distinguish sample rates >1 or <=1
• Modify squash gap to consider overlap. Plot overlaps in orange, gaps in red. Fix bugs off recalculate number of gaps when zoom in.
• Sorting all data by traces' start time.
• Remove obspy's merge which slow down performance and remove the overlaps in data. Instead, use np.hstack() to combine data. However, data will be split at overlaps to plot them separately to avoid graph going back and forth.
• Gather all final tasks to finalize_data() to serve in case of reading more than one folder.
• Change Reftek's mass position channel from MP to MassPos to avoid name that may be used in the future.
• Remove check box for Detecting Gaps. SOHView will do that when the Minimum Gap Length box is filled with a number.