Add 3D slant stack operators

[nmbader]

• Add two 3D slant stack operators, one working in the 3D Fourier domain (analogous to the existing 2D operator) and one working in the original time-space domain (shift and sum).
• Add 2D slant stack operator working in the time-space domain (shift and sum)
• Simplify and fix bug in the existing 2D slant stack operator
• Add unit tests

Note 1: in 3D with mode="depth", the mapping from subsurface offsets to aperture angle depends on the geologic dip.
Note 2: the shift and sum operator is typically slower than the Fourier domain operator. However, it yields result with less artifacts and can serve as a verification test. Moreover, the 3D shift and sum in mode="depth" can accommodate depth-varying geologic dip, which is not possible in the Fourier domain.

[samtkaplan]

Looks good. I'm wondering if WaveFD is a bit of a heavy dep to add, but I'm not sure I have a better suggestion at the moment.

[jkwashbourne-oss]

nice add

shift/sum allows non-regular locations but I guess we are assuming everything regular

[nmbader]

Looks good. I'm wondering if WaveFD is a bit of a heavy dep to add, but I'm not sure I have a better suggestion at the moment.

I will try a julia native package for shifting instead of WaveFD

[nmbader]

nice add

shift/sum allows non-regular locations but I guess we are assuming everything regular

yes, the shift/sum operator can take either regular offsets in xy or fully irregular offsets

[nmbader]

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[nmbader]

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