make sure to check for types
docs for Curve
added BoxCurve to create a box-shaped Curve object from two corners
removed BoxCurve and added documentation to DataBox (aliased to Box)
documenting Circle, Circles, Ellipse, Ellipses
documenting ErrorBarsX, ErrorBarsY, SymmetricErrorBarsX, SymmetricErrorBarsY
docs for PlotKye
add the plot_hist function to func.py, as well as helper function make_hist
remove debug statements
document the very useful but undocumented drop_to_zero keyword/attribute of Histogram
fix another bug checking symbol type
get rid of density keyword, just use norm
Merge pull request #35 from esheldon/master

make sure to check for types