v2.4.1

FINUFFT version 2.4.1

Small fixes:

• Fix cufinufft importing bug found by @fzimmermann89 (Barbone, issue #707, PR #708)
• Cached the optimal thread number (# physical cores) to reduce system call
  overhead in repeated small transforms (@YuWei-CH), #697, fixing #693).

New feature:

• Python simple CUDA API for type3 (@MaximEremenko)

Minor changes:

• new Python version of 2D Poisson solve tutorial (@juliusgh, #700).