Introduce RangeFilter and the corresponding operator "<=x<="

[KaiVolland]

This adds support for a between filter range filter which allows to check if a numeric property value is between to other values (within a range). e.g.:

// Filter all cities with a population between 5.000 and 25.000:
const filter = ['<=x<=', 'population', 5000, 25000];

targets geostyler/geostyler#87

It also fixes the samples.

[chrismayer]

Just for general interest: Should something like '<=x<' et al also be introduced?

[KaiVolland]

Just for general interest: Should something like '<=x<' et al also be introduced?

@chrismayer
Good question. Actually i don't think we should make this to complex. But maybe it would be better to make the range values inclusive. I think SLD handles it like that …
But id like to have a nicer operator then '<=x<=' 🤔

[KaiVolland]

Just for general interest: Should something like '<=x<' et al also be introduced?

@chrismayer
Good question. Actually i don't think we should make this to complex. But maybe it would be better to make the range values inclusive. I think SLD handles it like that …
But id like to have a nicer operator then '<=x<='

The mathematical correct symbol for ranges are brackets, which will completly conflict with the filter syntax. And as the geostyler-style won't be written by hand in most cases the '<=x<=' should be ok for now.