Fixes some minor bugs found in smtk/strong_motion_selector.py (just spotted via editor highlights, no tests implemented)
Decouples all plot data calculation from matplotlib rendering part: the old smtk.residuals.residual_plotter.py still handles the rendering (matplotlib), but a new module (smtk.residuals.residual_plots.py) implements the core functions, so that the code is cleaner and matplotlib is not imported when only plot data is required
Implements tests for both the two modules above (by mocking pyplot.show and other GUI-related functions)
This PR essentially does: