GeoTiff partial loading optimize for all roi

[HowcanoeWang]

if RAM >> GeoTiff.filesize and len(roi) > 100:
    full load GeoTiff, and delete the variable after finish.

Have test fulling loading into memory, DOM.dtype(uint8) == DSM.dtype(float32), 20w x 20w dom requires 91GB RAM. Not applicable.

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The optimal choice:

read one line tail for muitiple roi if applicable.

No need duplicately read geotiff for each roi.

> ROI.get_z_from_dsm():

1. calculate each roi and its bbox.
2. bbox to geotiff tail line & colume
3. revert previous index, to each geotiff tail, which columns are which roi and roi corresponding parts
4. For loop, for each geotiff tail, read to numpy, and crop & save to correspoing roi parts
    1. If one roi is completed loaded, return cropped results for calculation.
    2. delete the completed roi in memory.
    3. continue for the next tail.