Automatically chose eltype of OperatorConv weights

[tknopp]

I tried to replace FourierTransform with ChebychevTransform in OperatorKernel but this did not work since the later implicitly assumes complex weights in the OperatorConv. I changed it such that the eltype is derived from the Transform.

[codecov]

Codecov Report

Merging #84 (1a5b271) into main (60b7726) will decrease coverage by 0.84%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   90.17%   89.32%   -0.85%     
==========================================
  Files          10       10              
  Lines         173      178       +5     
==========================================
+ Hits          156      159       +3     
- Misses         17       19       +2     

Impacted Files	Coverage Δ
src/Transform/chebyshev_transform.jl	94.11% <0.00%> (-5.89%)	⬇️
src/operator_kernel.jl	98.27% <66.66%> (-1.73%)	⬇️
src/Transform/fourier_transform.jl	100.00% <100.00%> (ø)

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[yuehhua]

It looks good to me.