Floating-point underflow in root-finding functions

[UnkindPartition]

Root-finding functions like arkRootfind and cvRootfind use floating-point multiplication to check whether a sign change has occurred. This is prone to floating-point underflow (e.g. (-1e-166) * 1e-166 evaluates to 0) which makes the algorithm assume that the sign change was on the wrong sub-interval.

Here's a commit fixing this issue for cvode and arkode; feel free to pull it into sundials if you find it useful. The fix may need to be ported to other packages as well.

Thanks for maintaining sundials!

[eltix]

@balos1 Any chance this can be merged upstream? Do you want us to open a PR?

Thanks for maintaining Sundials :)