Access collections filter properties of collections

[flahn]

The function load_collection() offers the opportunity to filter within collections (maybe also over multiple collections) with certain defined properties. This general function was implemented here: #102

A statement can look like this:

list(
`eo:cloud_cover` = function(x) x >= 0 & x <= 30
)

Now the problem is currently that it is not really visible which property names are allowed. As stated here those information should reside in the collections summaries field. However those information is partly read during the visualization of a collection, but was not used elsewise. The user should be able to access the information which property names are 'allowed' for a certain collection.

[flahn]

We need probably also to wait for a change / clarification in the main project.