checkplot png: add features; defaults are the same
checkplot png: add features; defaults are the same

* add option to overplot circle on DSS maps (AKA: the aperture)

* add try/except logic for text safety

* add ability to change tick sizes, and marker sizes

* add returnfigure kwarg for twolsp_checkplot_png, which returns the
  matplotlib figure instance, rather than saving it to drive.

I've found the latter functionality helpful to make multi-page PDFs when
assessing a lightcurve, for which the twolsp_checkplot becomes one of a
number of pages.

lgbouma/astrobase,waqasbhatti/astrobase,lgbouma/astrobase,waqasbhatti/astrobase,lgbouma/astrobase,waqasbhatti/astrobase