Html and string representation of object both shown together in presentation mode

[TomNicholas]

Xarray has objects with both a HTML representation and a printable string repr. The HTML repr is implemented through a _repr_html_ method, and works great in jupyter normally, where it is the default way of displaying an xarray.Dataset object.

When I make a RISE slide with the same code, in presentation mode it first prints the string representation of the ds object, then displays a (very messed up-looking) version of the HTML repr.

It looks like an issue was raised for odd sizing of HTML output (#51), but here I want to raise the separate issue of printing both the string repr and the HTML repr.