nonlinz.cpp: use eigen instead of sparse13 #6764

Job	Run time
macOS-11 - setuptools (CC=gcc-9 CXX=g++-9)	30m 16s
macOS-11 - cmake (CC=gcc-10 CXX=g++-10)	28m 56s
macOS-11 - cmake (CC=gcc-10 CXX=g++-10)	29m 14s
macOS-11 - cmake (-DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	27m 17s
macOS-11 - cmake (-DNRN_ENABLE_MPI=OFF -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	25m 42s
macOS-11 - cmake (-DNRN_ENABLE_PYTHON=OFF -DNRN_ENABLE_RX3D=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-10 CXX=g++-10)	26m 34s
ubuntu-20.04 - setuptools (CC=gcc-9 CXX=g++-9)	4m 8s
ubuntu-20.04 - cmake (CC=gcc-10 CXX=g++-10)	4m 13s
ubuntu-20.04 - cmake (CC=gcc-10 CXX=g++-10)	4m 32s
ubuntu-20.04 - cmake (-DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	3m 14s
ubuntu-20.04 - cmake (-DNRN_ENABLE_MPI=OFF -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	2m 56s
ubuntu-20.04 - cmake (-DNRN_ENABLE_PYTHON=OFF -DNRN_ENABLE_RX3D=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-10 CXX=g++-10)	3m 5s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNMODL_SANITIZERS=undefinedundefined)	3m 15s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNMODL_SANITIZERS=addressaddress)	2m 41s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_MPI=OFF -DCORENRN_ENABLE_OPENMP=OFF -DNRN_ENABLE_RX3D=OFFthread)	3m 29s
macOS-12 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_RX3D=OFF -DNMODL_SANITIZERS=addressaddress)	6m 19s
Final CI	1s
	3h 25m 52s

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