BUG: `GanttSeries.from_pandas()` now fails on inherited validation

[hcpchris]

Following the upgrade to v.1.4, the GantSeries.from_pandas() method is now breaking when evaluating the truthiness of a NumPy array as outlined in this forum post.

[hcpchris]

This issue really needs to be addressed in Highcharts Core for Python. I've filed a separate bug report there: highcharts-for-python/highcharts-core#162.

[hcpchris]

Turns out that it was an error partially in Highcharts Gantt for Python and partially in Highcharts Core for Python. The root cause of the problem was that the GanttSeries._data_collection_class() method was actually returning the data point class, while the ._data_point_class() method was returning the collection class (they were swapped).

Fixing this issue actually uncovered a separate problem, namely that date validation fails when converting between numpy.datetime64 and datetime.datetime. I implemented utility_functions.datetime64_to_datetime in the Highcharts Core for Python library, until such time as the Validator Collection extends datetime validation coverage to support numpy.datetime64.