Extend filter functionality - Allow to filter by equal value

[santilland]

In order to be able to extract data more efficiently through the interface we need new types of filtering that are not currently possible through the range filtering.

Allowing filtering by equal value.
This is possible for numeric data (using the same min and max in the range filter) but it would be important especially for mie_wind_result_observation_type and rayleigh_wind_result_observation_type which are strings data to allow to filter by a specific string to be equal, e.g. "cloudy", "clear"

A possibility could be to pass an equal parameter in filter object instead of min max.