Replace Meschach/sparse13 libraires with Eigen

[alkino]

Meschach (http://homepage.divms.uiowa.edu/~dstewart/meschach/) and sparse13 (https://netlib.org/sparse/) are 2 old (1997, 1988) libraries for doing dense and sparse matrix computations.

We can maybe replace them by a more modern and extern eigen3 library (https://eigen.tuxfamily.org).

For Meschach, it will be needed to rewrite the really easy wrapper: https://github.com/neuronsimulator/nrn/blob/master/src/ivoc/ocmatrix.h RX3D only seems to include Meschach directly.

• [x] Meschach
• [ ] sparse13:

Sparse13 is used three times:

• [ ] kschan: https://github.com/neuronsimulator/nrn/pull/2489
• [x] nonlinz: https://github.com/neuronsimulator/nrn/pull/2623
• [ ] treeset: https://github.com/neuronsimulator/nrn/pull/2643