Add Cardinality enum

[Stiivi]

Add Cardinality or CardinalityEstimation enum:

class CardinalityEstimation(Enum):
    tiny = 0
    low = 1
    medium = 2
    high = 3

From the Level class documentation:

• tiny – few values, each value can have it's representation on the screen, recommended: up to 5.
• low – can be used in a list UI element, recommended 5 to 50 (if sorted)
• medium – UI element is a search/text field, recommended for more than 50 elements
• high – backends might refuse to yield results without explicit pagination or cut through this level.

Affected places:

• Level class
• Dimension class
• Drilldown.high_cardinality_levels()