fix toctrees warnings during documentation generation

[sdrave]

There seems to be a clash between numpydoc and autosummary, resulting in over 8000 warnings of the form:

<autosummary>:None: WARNING: toctree contains reference to nonexisting document 'generated/pymor.discreteoperators.basic.GenericLinearOperator.apply'

[sdrave]

The warnings are eliminated if the following lines of code are commented out in numpydoc.docscrape_sphinx.SphinxDocSting._str_member_list():

if autosum:
    out += ['.. autosummary::', '   :toctree:', '']
    out += autosum

[sdrave]

Much better now! However still a slight annoyance since the real warnings are mixed with the toctree warnings, so I remove the milestone tag but leave the bug open for now.

[sdrave]

With bd5f415, documentation now builds without warnings. Still looks horrible, however.