Frontal solver example

It might be a fun project to implement a frontal solver to see where we are to develop specialized linear solvers. A starter should be the original paper by Irons [1] which contains a fortran code which we could port.

## References

[1] Irons, Bruce M. (1970). "A frontal solution program for finite element analysis". International Journal for Numerical Methods in Engineering. 2 (January/March): 5–32. [Bibcode](https://en.wikipedia.org/wiki/Bibcode_(identifier)):[1970IJNME...2....5I](https://ui.adsabs.harvard.edu/abs/1970IJNME...2....5I). [doi](https://en.wikipedia.org/wiki/Doi_(identifier)):[10.1002/nme.1620020104](https://doi.org/10.1002%2Fnme.1620020104).