We have seen quite some issues with Eigen-based solvers when it comes to OpenACC/OpenMP offload on GPUs. In order to simplify the implementation and reduce the maintenance burden, we should evaluate and then remove Eigen dependency from NMODL.

As part of this ticket:

• Check the existing of Eigen
• Confirm if we can replace the usage by Crout solver
• Have implementation without Eigen
• Measure performance against standard benchmarks - channel-benchmark, olfactory-bulb and reduced-dentate
• Merge the implementation!

Note that some was already done in #690.