use isfresh like mechinisim for precs

[oscardssmith]

alternative to #535, fixes #527 for real this time. I don't love this answer, but I think it is the easiest way for now.

[j-fu]

I think we could keep the reuse_cache keyword, which is the nicer API.
In common.jl L264 we could do

function SciMLBase.solve!(cache::LinearCache, args...; reuse_precs=cache.precsisfresh, kwargs...)
    cache.precsisfresh=reuse_precs
    solve!(cache, cache.alg, args...; kwargs...)
end

[j-fu]

Or maybe this could go into reinit! ?

function SciMLBase.reinit!(cache::LinearCache;
                           A = nothing,
                           b = cache.b,
                           u = cache.u,
                           p = nothing,
                           reinit_cache = false,
                           reuse_precs = false)
    (; alg, cacheval, abstol, reltol, maxiters, verbose, assumptions, sensealg) = cache


    isfresh = isnothing(A)
    precsisfresh =  ( isfresh || isnothing(p) ) && !reuse_precs

Appears to me a bit more consistent, but not sure.

[oscardssmith]

good to merge?

[j-fu]

oscardssmith#4 adds a test for for working reuse_precs

[oscardssmith]

thanks!

[j-fu]

LGTM