FEATURE: read only valid portion of imagery

[remtav]

Is your feature request related to a problem? Please describe.
Problem: some mosaics we use consists mostly of nodata regions. For example, NB_Edmunstron_2016-B:

This 15 cm image covers approx. 24 x 19.5 km -> 468 km2 including nodata (5200 km2 in 50 cm equivalent). The file for each indivial band itself is over 28Gb. However, the valid portion of this image covers appox. 10-15 % of the entire image.

How can we optimize this?

@mpelchat04 had brought up this issue already... The only solution we could come up with was creating smaller mosaics.

Describe the solution you'd like
How can nodata be supported along the way, in the dataloader for example? Anywhere else? Any other ideas?

[remtav]

Update: this remains a major optimization issue for our extraction pipeline :)

Other examples where valid data is less than 75%: