Replace Meschach by eigen #6527

Job	Run time
macOS-11 - setuptools (CC=gcc-9 CXX=g++-9)	17m 57s
macOS-11 - cmake (CC=gcc-10 CXX=g++-10)	17m 59s
macOS-11 - cmake (CC=gcc-10 CXX=g++-10)	18m 9s
macOS-11 - cmake (-DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	15m 27s
macOS-11 - cmake (-DNRN_ENABLE_MPI=OFF -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	12m 36s
macOS-11 - cmake (-DNRN_ENABLE_PYTHON=OFF -DNRN_ENABLE_RX3D=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-10 CXX=g++-10)	9m 30s
ubuntu-20.04 - setuptools (CC=gcc-9 CXX=g++-9)	11m 56s
ubuntu-20.04 - cmake (CC=gcc-10 CXX=g++-10)	14m 42s
ubuntu-20.04 - cmake (CC=gcc-10 CXX=g++-10)	13m 35s
ubuntu-20.04 - cmake (-DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	17m 35s
ubuntu-20.04 - cmake (-DNRN_ENABLE_MPI=OFF -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	12m 32s
ubuntu-20.04 - cmake (-DNRN_ENABLE_PYTHON=OFF -DNRN_ENABLE_RX3D=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-10 CXX=g++-10)	6m 44s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNMODL_SANITIZERS=undefinedundefined)	13m 54s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNMODL_SANITIZERS=addressaddress)	13m 8s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_MPI=OFF -DCORENRN_ENABLE_OPENMP=OFF -DNRN_ENABLE_RX3D=OFFthread)	31m 27s
macOS-12 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_RX3D=OFF -DNMODL_SANITIZERS=addressaddress)	13m 35s
Final CI	1s
	4h 0m 47s

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