fix plot_std_diffs, add bal_tol, condense to one plot

[ras44]

Proposed changes

This PR fixes a variety of issues in visualize.py related to the functions plot_ps_diagnostics and _plot_std_diffs. Previously, when plotting, no range values/ticks were displayed on the x-axis and the displayed point locations of the IPW standardized diffs in the seaborn plots did not reflect the correct values. This PR also adds a bal_tol argument to adjust the tolerance for what is considered balanced. Finally, the two pre/post plots have been condensed into one plot with hue identifying the pre/post locations of the std diffs:

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[ras44]

FYI, this type of plot is commonly known as a love plot, so an alternative to modifying plot_ps_diagnostics may be to add a new function called love_plot to visualize.py which produces the above image. Nevertheless, currently it appears that the dual-plot image produced by plot_ps_diagnostics does not display results correctly.

[jeongyoonlee]

LGTM. Thanks!