Allows symmetric and hermitian options for all PreallocatedLinearOperator methods

[amontoison]

In Krylov.jl, we cannot use Symmetric matrices because PreallocatedLinearOperator(M ::Symmetric{T}; storagetype, symmetric) is not implemented.

This PR allows the use of symmetric and hermitian options even if they are not used with Symmetric, Hermitian and SymTriadiagonal wrappers.

https://github.com/JuliaSmoothOptimizers/Krylov.jl/blob/master/src/variants.jl#L24
https://github.com/JuliaSmoothOptimizers/Krylov.jl/blob/master/src/variants.jl#L62
https://github.com/JuliaSmoothOptimizers/Krylov.jl/blob/master/src/variants.jl#L65
https://github.com/JuliaSmoothOptimizers/Krylov.jl/blob/master/src/variants.jl#L73
https://github.com/JuliaSmoothOptimizers/Krylov.jl/blob/master/src/variants.jl#L76

[codecov]

Codecov Report

Merging #165 (90c807c) into master (4eb3c4f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #165   +/-   ##
=======================================
  Coverage   96.92%   96.92%           
=======================================
  Files          14       14           
  Lines         748      748           
=======================================
  Hits          725      725           
  Misses         23       23           

Impacted Files	Coverage Δ
src/PreallocatedLinearOperators.jl	96.77% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4eb3c4f...cce17d0. Read the comment docs.

[amontoison]

I think that it's more relevant to use the multiple dispatch of the PreallocatedLinearOperator method instead of adding variants in the src/variants.jl file of Krylov.jl.

[amontoison]

@dpo is it fine if I merge this pull request ?