`WSIReader` support for reading by mpp or magnification

[bhashemian]

In MONAI Pathology WG, we have discussed about the need for being able to read whole slide images at specific resolution (mpp or magnification) in addition to reading at a specific level. The feature request has been made by @cooperlab in cucim repo rapidsai/cucim#389.

I have discussed with cuCIM team about this topic and the conclusion was that it may be more useful to implement this functionality in MONA rather than in cucim for few reasons:

• this feature will be automatically supported for all the backends (cuCIM, OpenSlide, TiffFile, etc)
• the changes in magnification can be tracked in MONAI and be used to invert the process

To achieve this, the following functionalities need to be added to MOANI WSIReader:

• to find the next higher level to the requested mpp or magnification
• load the requested image that level with the appropriate pre-resized size
• resample/resize the image to the target mpp/magnification

[bhashemian]

@cooperlab what do you think?

[cooperlab]

@drbeh LGTM. The guiding principles here are that developers shouldn't have to do a lot of calculations to determine what they have or what's been done to it, and to make clear what definitions like location correspond to.