Added support for reading multiple molecules

Currently done with an open function, support for writing as well as
strings and streams pending. Tested with xyz which works for multiple
molecules without further modification, and MDL which required minor
modification to support SDF (skipping record separators).

Change-Id: Ia81671ac2bafe576f1834b93796f6075f9610104
Added some support for writing multiple molecules

Added a new value to the Operation enum for MultiMolecule, this should
be used to indicate whether formats support multiple molecules. Used XYZ
and SDF as the test formats again.

Change-Id: I9672735fc14cc159a04f3a2eeb1260e001edc00b
Fix filebrowsewidget test for windows.

Qt just checks extensions to determine if a file is executable
on windows, and .py is not recognized. Use the standard cmd.exe
file to test the widget.

Change-Id: I8c097a0c996153e27e4bedaabe20cc9dd8508cbc
Merge topic 'multi-molecule' into master

c6bc1180 Added some support for writing multiple molecules
4156fa17 Added support for reading multiple molecules
Merge topic 'windows-executables' into master

6ed2bb90 Fix filebrowsewidget test for windows.