Use Spectra/Eigen as a replacement for ARPACK

[drroe]

The only functionality in CPPTRAJ that requires Fortran is ARPACK. Spectra/Eigen is a potential C++ replacement:

https://spectralib.org/

https://libeigen.gitlab.io/

Both libraries are MPL2, which I think is compatible with the GPL3 license that CPPTRAJ uses.

I envision this being handled the way that CPPTRAJ currently handles FFT - use FFTW if present, otherwise fall back to the Fortran code.