Fix empty columns for missing data

In some cases read() would return a completely empty dataframe without column name if a tag has no data in the timeframe specified. This would happen even for completely valid tags.

This release improves the situation somewhat as it should guarantee that a column with the tagname is included in the result. The content of the column can consist of None, NaN or be completely empty. The result depends on the handler, whether the tag is queried alone or alongside other tags that do have data for the time frame, and even the tag itself.

At least for now it is up to the user to handle these various cases appropriately.

Fixed

• Improved return values from read() for tags with no data in specified time frame.
• Added requests_ntlm as requirement.