Better data type detection for pre_aggregated, indexed dataframes

[dorisjlee]

When a dataframe is pre-aggregated, our type detection based on cardinality often fail to detect the type correctly. For example, when the dataset size is small (often the case when data is pre-aggregated), nominal fields would get recognized as a quantitative type.

df = pd.read_csv("lux/data/car.csv")
df["Year"] = pd.to_datetime(df["Year"], format='%Y') # change pandas dtype for the column "Year" to datetype
a = df.groupby("Cylinders").mean()

a.data_type

As a related issue, we should also support the detection of types for named index, for example, in this case, Cylinders is an index, so its data type is not being computed.

[dorisjlee]

Closing this after #287 is merged in. Great work Kunal!