checkplot: add a user configurable zoomed-in phased LC plot view

[waqasbhatti]

This will be a keyword option to checkplot_png and checkplot_pickle, useful for zooming in to show transits, etc.

[waqasbhatti]

Now fixed. The new behavior is achieved using the xliminsetmode kwarg, like so:

x = checkplot.checkplot_pickle(
    [lsp,pdm], 
    lcd['rjd'], lcd['atf_000'], lcd['aie_000'], objectinfo=lcd['objectinfo'], 
    xliminsetmode=True, plotxlim=[-0.2,0.2], sigclip=[10.0,4.0]
)

This works for checkplot_pickle, checkplot_png, and twolsp_checkplot_png, and makes something like: