Make block precon builders mutable

Blocksizes & partionings sometimes not known when creating the strategy.

Author: j-fu

src/factorizations/blockpreconditioner.jl

@@ -112,7 +112,7 @@ from partition of unknowns.
 
 - `precs(A,p)` shall return a left precondioner for a matrix block.
 """
-Base.@kwdef struct BlockPreconBuilder
+Base.@kwdef mutable struct BlockPreconBuilder
     precs=UMFPACKPreconBuilder()
     partitioning= A -> [1:size(A,1)]
 end

src/preconbuilders.jl

@@ -26,7 +26,7 @@ struct JacobiPreconBuilder end
 Return callable object constructing a left zero fill-in ILU preconditioner 
 using [ILUZero.jl](https://github.com/mcovalt/ILUZero.jl)
 """
-Base.@kwdef struct ILUZeroPreconBuilder
+Base.@kwdef mutable struct ILUZeroPreconBuilder
     blocksize::Int = 1
 end