bugfixes with Fourier slantstack - accelaration of 3D shiftsum - Unit tests

[nmbader]

jop_slantstack updates:

• Correct the mapping formula in the 3D wavenumber domain in the presence of geologic dip
• Correct the out-of-bounds wavenumbers check for all Fourier operator (previous implementation was incorrect beyond the aliased wavenumbers)
• Speed up the ShiftSum operator in 3D when depth variable dip is present by replaced the Shifting operation by direction Interpolation of the appropriate sample
• Improve multithreaded double loops and reducing local array allocations
• Robustify unit tests and add new ones

[jkwashbourne-oss]

looks great, thanks for the improvements