expect() implementation and addition of waveguide operators

[mabuni1998]

expect(O,rho), with O being related to a waveguideoperator, now returns a sum over all time bins.

Also added the ability to add two waveguide operators by creating a lazysum:

w = destroy(bw)
wd = create(bw)
S = w + wd

See also: #44

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (f6a0023) 58.11% compared to head (4dfc5b3) 57.27%.
Report is 5 commits behind head on main.

Files	Patch %	Lines
src/WaveguideInteraction.jl	0.00%	24 Missing ⚠️
src/WaveguideOperator.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   58.11%   57.27%   -0.84%     
==========================================
  Files          11       11              
  Lines        1664     1690      +26     
==========================================
+ Hits          967      968       +1     
- Misses        697      722      +25     

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