mathematical notation bugs in docs

[ahbarnett]

Dear Tobias & crew,

I'm very glad you have mathematical descriptions of the transforms - this is essential for a professional math library.
But there are a bunch of typos in
https://github.com/JuliaMath/NFFT.jl/blob/master/docs/src/background.md
Mostly this is bold vs italic,
but also [-1/2,1/2) etc.

You may want to explain that there are other definitions,
eg
https://finufft.readthedocs.io/en/latest/math.html
and to tell the user to rescale their NU points to change definition.

I'll try to do a PR but may not be able to get to it :)

Keep up the good work. Best, Alex

[tknopp]

Bold vs italic is a workaround since the math engine did not render the italic at some places...
Will fix that. I will definitely reference https://finufft.readthedocs.io/en/latest/math.html and discuss the scaling part.

No need to make PR at this point. Better would be if you could review the text when I finished the documentation (its work in progress).

One thing I stumbled upon when reading FINUFFT documentation is that you seem to support odd N. We need to reflect that in the mathematics. FINUFFT will actually be the only implementation supporting this right now.