clean up preprocessing and add functions for modifying c, b, h

[chriscoey]

fixes #95, useful for new MOI Pajarito

[codecov]

Codecov Report

Merging #792 (9ea5ad6) into master (5352664) will decrease coverage by 0.12%.
The diff coverage is 93.96%.

@@            Coverage Diff             @@
##           master     #792      +/-   ##
==========================================
- Coverage   96.70%   96.57%   -0.13%     
==========================================
  Files          54       54              
  Lines        8218     8353     +135     
==========================================
+ Hits         7947     8067     +120     
- Misses        271      286      +15     

Impacted Files	Coverage Δ
src/MathOptInterface/wrapper.jl	82.82% <75.00%> (-0.94%)	⬇️
src/Solvers/process.jl	93.57% <95.32%> (+0.52%)	⬆️
src/Solvers/Solvers.jl	92.21% <95.68%> (+0.43%)	⬆️
src/MathOptInterface/cones.jl	96.22% <100.00%> (ø)
src/Solvers/systemsolvers/common.jl	100.00% <100.00%> (ø)
src/Solvers/systemsolvers/qrchol.jl	99.18% <100.00%> (+0.01%)	⬆️
src/linearalgebra/dense.jl	62.50% <0.00%> (-5.56%)	⬇️
src/Cones/hyporootdettri.jl	99.48% <0.00%> (+0.51%)	⬆️

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