Add missing dfsu read x,y test

[ecomodeller]

Dfsu2DH.read(x=, y=) can be used to read a number of positions (stations).

Inspired by discussion in #482

@Hendrik1987 is this good enough, or would you still prefer a extract_stations method?

[Hendrik1987]

@ecomodeller :
Thanks for keeping our old discussions in mind :-)

If your solution under the hood avoids reading all elements or in some other way handles larger than memory files, then this is amazing! I think, no need to introduce a dedicated extract_station method.

I rather think it could be useful to also here include a n_nearest argument, allowing to use the interpolation methods on the ds afterwards. This would result in loading also these neighbouring elements in addition to the ones in your plot above. In fact, the current ds interpolation does not define a max distance, or does it? I.e., blindly interpolating on the subset you show above with default n_nearest, would lead to unexpected restults.

[Hendrik1987]

@irnigerandrea: Don't you think this could be useful for us?