milliseconds are not supported

[wrohdewald]

testing current master

GPX 1.1 explicitly allows milliseconds.

to_xml() uses DATE_FORMAT without milliseconds by default, losing information. I don't mind if that is the default but then the documentation should say so.

parse_time always ignores milliseconds even when using DATE_FORMATS[1] because everything after the dot is already removed before parsing:

if '.' in string:
    string = string.split('.')[0]