Breaking: Standardise cf standards and missingval handling

[rafaqz]

This PR standardises cf and missingval behaviour accross all sources.

New keywords are:

• cf=true: apply offset and scale if the are not zero and one.
• maskingval=missing this is the value the missingval will be converted to if there is one.

I don't like either of these names! if you can think of anything better please suggest something.

@felixcremer @meggart this is the new diskarrays object that does 2-way cf and missingval/maskingval conversions

https://github.com/rafaqz/Rasters.jl/compare/cf?expand=1#diff-1478a88a8bdcffcf7f556cc1865dbe18d69b5c03f75b4481e1f5889e8a6b6b73

[Rapsodia86]

How about inverting it? Instead of "cf" call it "raw". If raw=false, offset and scale are applied, if raw=true, offset and scale are NOT applied?

And raw=true would be the default?

Maybe "navalue" instead of "maskingval"?

[rafaqz]

Currently I've got scaled=true as the default instead of cf. raw might suggest the missing value is also not transformed?

Main concern with navalue is it sounds like R, and isn't really used in julia

[Rapsodia86]

Yes, you are right that with "raw" one may expect that data were not touched at all.

Indeed, "navalue" originates from my heavy R usage! And I agree NA is not very julia way.

Point taken:)

[rafaqz]

Thanks for the input though, good to have all the options to choose from

[rafaqz]

Coming back to this I like how short and obvious raw=true is!

But to mean no scaling and no masking - so it will override everything else.

[asinghvi17]

When should I use this as opposed to Raster(data, dims; ...)? What are the advantages beyond efficiency in writing to file?

[asinghvi17]

Or is the direct constructor going to call create?

[rafaqz]

Well Raster(filename) reads an existing file... create(filename) creates one.

So this is best for building something new from scratch, Raster is best for opening something, rewrapping a DimArray, etc. Otherwise you need to define the Raster in-memory then write it as separate steps.

(create is already called in a lot of places under the hood where a new file might be created, like in nearly all methods with a filename keyword. It's been here for years, just not publicly)

[asinghvi17]

Gotcha, I guess I'm not entirely clear on what to use when creating a purely in memory dataset. Does the pure Raster(data, dims; kwargs...) constructor have the same options? Should it? That's the most intuitive option IMO to create an in memory raster...

[rafaqz]

Yeah it will be pretty much identical for in-memory rasters. But if you want chunks and e.g. compression options you have to do it separately in write. An in-memory raster doesn't have chunks or options like that.

[asinghvi17]

Sweet that sounds good. Will add a suggestion for a line in the docstring to clarify.