Feature/add cont cat variable type

[CMobley7]

This PR adds whether an indicator is a continuous or categorical variable. I added this for most momentum, overlap, trend, volatility, and volume indicators. This information should be helpful to ML practitioners. Please let me know if any changes are necessary.

[CMobley7]

This gives an additional solution to #434

[CMobley7]

Closing because this doesn't actually work, likely due to how the final df is constructed. Also, it's easy to just do something like

def get_categorical_columns(df, threshold=10):
    """Identify columns with unique values below a certain threshold."""
    categorical_columns = []

    for col in df.columns:
        if len(df[col].unique()) <= threshold:
            categorical_columns.append(col)

    return categorical_columns