rowstart for PlusOp/TimesOp for InfiniteCardinal

[jishnub]

Also, retain the Conversion operators while multiplying TimesOperators, which helps with consistency.

[codecov]

Codecov Report

Patch coverage: 91.66% and project coverage change: -2.50 ⚠️

Comparison is base (441f15c) 71.31% compared to head (81b22a6) 68.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   71.31%   68.81%   -2.50%     
==========================================
  Files          81       81              
  Lines        8533     8492      -41     
==========================================
- Hits         6085     5844     -241     
- Misses       2448     2648     +200     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	79.16% <ø> (-4.84%)	⬇️
src/Operators/general/algebra.jl	85.45% <87.50%> (-5.44%)	⬇️
src/LinearAlgebra/clenshaw.jl	35.26% <100.00%> (-1.06%)	⬇️
src/Multivariate/Multivariate.jl	52.54% <100.00%> (-1.85%)	⬇️

... and 46 files with indirect coverage changes

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