Fix filters

[m-mohr]

Currently, the property filters (e.g. filter or properties in load_collection) is completely broken as there's no way to properly reference the metadata fields and also the type "boolean" for expression elements doesn't really reflect that you can insert processes returning a boolean.

How I could imagine it working in the future: #84 (comment)

[lforesta]

the property filters (e.g. filter or properties in load_collection)

"filter" here is the process filter? I thought this process does not work on metadata but on pixel values?

[m-mohr]

filter (and filter_*) are working on dimension labels, not pixel values.
The parameter properties in load_collection is working on metadata.
I haven't made this clear in the initial post. Anyway, there's no way to reference the dimension labels/metadata fields, which is an issue at the moment.

[m-mohr]

Closing as duplicate of #84