Enable read-only NetCDF DAP uri:s

[juselius]

Prevent (local) file existence checks for NetCDF uri:s which start with http. Instead pass directly to nc_open_chunked(), to enable read-only DAP/THREDSS support.

[juselius]

Sorry for modifying white space. My editor removes them, and was too tired to put them back.

[vassilyl]

@juselius, thank you for your interest in contributing to SDSlite. The solution you suggest will prevent treating properly files named http.nc and alike. Can you pls create an issue that documents current behaviour with DAP urls?

[juselius]

Hi @vassilyl, I'll provide a better solution soon, so that it's done properly. The problem is that I'm quite rusty in C#, since I work mostly in F# on .NET. I already tried to implement a DapUrl property in the URI, but there we so many levels of inheritance and so little time, that I just hacked the FileName property instead to get a working solution.

[juselius]

@vassilyl is it better now? I have added a Url property for DAP access to the Uri, and refactored things a bit to avoid duplication.