TFP: Shell Pandas operator classes

[ianspektor]

[WIP] Depends on #156 's formatting configs to reduce formatting diffs.

Initial folder structure for operator definitions. Defined:

• base Operator and WindowOperator classes
• example AssignOperator and SimpleMovingAverage shell classes

For simplicity at this point, and given we are going to be using pandas for the MVP, we decided to create these aliases:

• Interval as an alias of pd.Timedelta
• Sampling as an alias of pd.MultiIndex (with the restriction of the last level of it being a DatetimeIndex)
• EventSequence as an alias of pd.DataFrame

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