fix: timestamps being returned as strings, and with wrong column header

[drake-nominal]

• If user requests absolute timestamps, returns pandas timestamps rather than iso 8601 strings
• if user under-requests tags, the error message is more intuitive
• if user over requests tags, column names aren't removed from the dataframe, but just filled with nans
• if there is no data for a batch of channels in a given timeslice, the channels aren't ignored from the dataframe, but just filled with nans
• If the timestamp column was renamed, undo this renaming and keep the index consistently named timestamps