Implement multiwavelet operators

[yuehhua]

Closes #29

[codecov-commenter]

Codecov Report

Merging #30 (58a290b) into master (678bd2b) will decrease coverage by 29.92%.
The diff coverage is 63.06%.

@@             Coverage Diff              @@
##            master      #30       +/-   ##
============================================
- Coverage   100.00%   70.07%   -29.93%     
============================================
  Files            2        5        +3     
  Lines           52      274      +222     
============================================
+ Hits            52      192      +140     
- Misses           0       82       +82     

Impacted Files	Coverage Δ
src/wavelet.jl	37.33% <37.33%> (ø)
src/polynomials.jl	73.55% <73.55%> (ø)
src/utils.jl	88.46% <88.46%> (ø)

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[tknopp]

@yuehhua: What is the status of this? Is this already working and can be used as a drop-in replacement for the FNO?

[yuehhua]

Sorry, it has been a long time without update. I will continue working on it these days.