Discover parameter: multifrequency and frequency

[abenassi]

• If the time index values are directly parsed into datetime.datetime it wouldn't be difficult to infer the frequency from a sample of the first time values.
• Strategies to infer frequency from more complex string time values should be attached to strategies to discover time_multicolumn, time_alignment, time_header_coord since it's part of the same problem: understand the structure of the time index and gather parameters about it that other strategies can have for granted to operate.