Add a function to read BRT (Basisregistratie Topografie)

[OnnoEbbens]

This PR adds a function to read BRT data within an extent. There is already functionality to read the BGT (Basisregistratie Grootschalige Topografie) but the BRT (BasisRegistratie Topografie) contains different data. It is hard to find a good distinction between the two, according to Wikipedia the BRT will be a subset of the BGT in the future but that is not the case yet.

For now I used the BRT to get line elements of waterbodies. In the BGT these are polygons.

[OnnoEbbens]

This PR is ready for review. Some remarks:

• The functions are very similar to the one in the bgt module but many details differ because the returned xml data does not have the same format/names/types. Therefore I would not try to merge the two modules even though some functions are exactly the same. Also because having completely separate modules is nice for flexibility to future changes in the xml formats.
• There is a function get_brt and get_brt_ogc. The latter one is way faster and a lot less code. However it does not work for extents with over a 1000 features and is therefore not very useful in nlmod. I did leave it in (with a warning if you request over 1000 features) because this may change in the future but I could take it out if that is better.
• There is also a layer in the BRT with 'relief' data. I do not read this layer because we will probably never use it. If you use layer='all' the 'relief' layer is skipped with a warning.

[OnnoEbbens]

failing tests not related to this PR