add NFFTNormalOp util functions

[JakobAsslaender]

Hi, I just added a few utility functions for the NFFTNormalOp and removed the requirement that tr is of type Trajectory, which makes it harder to use of the nodes key argument and has, to the best of my knowledge, no real benefit.

[codecov-commenter]

Codecov Report

Merging #44 (7b7af47) into master (aa3603b) will decrease coverage by 0.14%. The diff coverage is 12.50%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   63.77%   63.63%   -0.15%     
==========================================
  Files          63       63              
  Lines        3034     3041       +7     
==========================================
  Hits         1935     1935              
- Misses       1099     1106       +7     

Impacted Files	Coverage Δ
src/Operators/NFFTOp.jl	32.81% <12.50%> (-4.03%)	:arrow_down:

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[tknopp]

Would it be possible using your new toeplitz code here, which would give us ND for free?

[JakobAsslaender]

Yeah, we should do so and remove the Toeplitz implementation here to avoid two sets of conflicting code. I'll open an issue for us to remember.