Draft a ConcretizedOperator

[gaurav-arya]

TODO:

• make sure sparse matrices correctly handled (don't drop zeros)

[codecov]

Codecov Report

Merging #213 (508da8c) into master (dd8fc48) will decrease coverage by 0.17%.
The diff coverage is 53.44%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   70.24%   70.07%   -0.17%     
==========================================
  Files          10       10              
  Lines        1583     1641      +58     
==========================================
+ Hits         1112     1150      +38     
- Misses        471      491      +20     

Impacted Files	Coverage Δ
src/SciMLOperators.jl	100.00% <ø> (ø)
src/basic.jl	70.78% <50.00%> (-0.95%)	⬇️
src/interface.jl	52.25% <62.50%> (+0.79%)	⬆️
src/matrix.jl	67.85% <100.00%> (+0.33%)	⬆️

... and 1 file with indirect coverage changes

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[vpuri3]

can you explain what's going on?

[gaurav-arya]

If L is a lazy operator, we might want to do A = convert(AbstractMatrix, L) because cerrtain operations might be faster with the matrix. But written like I just did, A loses the updating behaviour, whereas ConcretizedOperator does not. Similar to _concrete_form for WOperator's.

This PR is still WIP though