Change to line_types parameter of base.assert_lines_of_type()

[marty-larocque]

Currently, base.assert_lines_of_type() only accepts a list of strings for the line_types parameter.

It would be simple and convenient to change it to also accept a single string. This would be useful if only one type of line needs to be checked for.

[marty-larocque]

@lwasser this can be closed when #239 is merged.

[nkorinek]

This can be closed, fixed in #274 @lwasser

[lwasser]

closing!