Some small performance optimizations

[JoshuaLampert]

Running (see Discourse thread)

test_function = (x) -> sin(x[1]^2 + x[2]^2)
npoints = 1000
points = rand(2,npoints)

using KernelInterpolation

nodeset = NodeSet(transpose(points))
kernel = PolyharmonicSplineKernel{dim(nodeset)}(1)
ff = test_function.(nodeset)
itp = interpolate(nodeset, ff, kernel)
##
using BenchmarkTools
@benchmark itp(pt) setup=(pt=rand(2))

on main:

julia> @benchmark itp(pt) setup=(pt=rand(2))
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  50.736 μs …  15.490 ms  ┊ GC (min … max):  0.00% … 99.18%
 Time  (median):     57.234 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   71.910 μs ± 316.812 μs  ┊ GC (mean ± σ):  12.60% ±  2.96%

     ▂  █▅  ▁                                                   
  ▂▂▄█▆▇██▇███▄▃▃▃▃▂▂▂▂▂▂▂▂▂▁▂▂▁▁▂▁▁▁▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂ ▃
  50.7 μs         Histogram: frequency by time         99.2 μs <

 Memory estimate: 86.27 KiB, allocs estimate: 2014.

and on this PR:

julia> @benchmark itp(pt) setup=(pt=rand(2))
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  14.906 μs …  16.043 ms  ┊ GC (min … max):  0.00% … 99.59%
 Time  (median):     16.838 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   23.739 μs ± 216.162 μs  ┊ GC (mean ± σ):  12.81% ±  1.41%

  ▂▂▆▇██▇▅▃▁▁                         ▁▁ ▁▂▃▅▄▄▄▄▃▂▂▃▃▂▂▂▂▁    ▂
  ████████████▆██▇▇██▇██▆▅▅▅▂▅▄▅▄▃▂▃▃▅███████████████████████▇ █
  14.9 μs       Histogram: log(frequency) by time      34.2 μs <

 Memory estimate: 31.45 KiB, allocs estimate: 1009.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.43%. Comparing base (69a8cc4) to head (1e534e2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/kernels/radialsymmetric_kernel.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   98.50%   98.43%   -0.08%     
==========================================
  Files          46       46              
  Lines        1275     1277       +2     
==========================================
+ Hits         1256     1257       +1     
- Misses         19       20       +1     

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