Adds a Durand-Kerner polynomial solver to core and improves solver API

[kennyweiss]

Summary

• This PR improves Axom's polynomial solver capabilities
• It adds a Durand-Kerner polynomial solver, which returns the complex roots for a univariate polynomial
• It also modernizes the API for the linear, quadratic and cubic solvers to use axom::ArrayViews instead of pointers to C-style arrays. The latter have been deprecated
• Misc: It also adds axom::Array::pop_back() for compatibility with std::vector

Durand-Kerner solver

Here's a visualization (courtesy of claude) of how the solver works for the polynomial: $z^3-1$, initialized with three seed points near the unit circle -- powers of $.4+.9i$

[BradWhitlock]

I noticed that the tparam doc says ScalarType is double or Complex. Is a static_assert() for that in order here, or might be useful to pass Vector<double,3> or other things in down the road?

[kennyweiss]

I added a static_assert. If we need more flexibility later, we can modify the function/signature.

[jcs15c]

Is there anything special about this particular seed value? Could there ever be utility to letting a user pick the seed value? I know that can help a lot in regular Newton solvers.

[kennyweiss]

It was the value on the wikipedia page and they explicitly say it's not special :)

Sure -- we can pass update the API to optionally pass in a seed.

[kennyweiss]

Done.

[jcs15c]

Looks good to me!