Contour does not pass a list of linestyles to LineCollection

[mdboom]

This is a possible fix for #1701. The LineCollection constructor's linestyle/linestyles kwarg expects a list of styles, but contour was passing it a single value.

[mdboom]

@efiring may want to look at this.

[mdboom]

Also: this will need a unit test before merging.

[efiring]

It looks like that helps; but the whole business of setting dash patterns (a tuple within a tuple), together with the question of when one needs a list versus when one can get by with a single value, remains a confusing mess. And if one makes an error, it can trigger an incomprehensible PYCXX error message.
It looks like LineCollection.set_linestyle() works with a single value (as implied by the name) for strings, but not for those ugly dash specifications. In retrospect, we probably should have chosen a string syntax for the dash specification, so that linestyle(s) would always be a string or a sequence of strings. That would also allow a simpler specification for the usual case in which one doesn't need the offset part of the specification.

[dmcdougall]

Superceded by #1718.