[Lens] Throw error on array values in formulas

[wylieconlon]

Formulas can't handle array valued data, specifically coming from last_value functions. The math function is not allowed to return array values, so formulas also cannot return array values. It should be treated as an error to evaluate a formula on an array value column.

Steps to reproduce:

1. Use the ecommerce sample data
2. Type the formula last_value(products.base_price)
3. Get the error [lens_merge_tables] > [mapColumn] > [math] > Objects must have a type property

The error message here should be much more obviously caused by array valued fields.

Another option to consider in the long term is allowing the last_value function to take a reduce/concat function such as last_value(products.base_price, reduce='average')

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

+1 for short-term (7.14) improving the error message and long term providing a way to work around it