Supporting augmented matrices directly in the fixed matrix input

[sangwinc]

This is to separate out this suggestion from issue #1848 , as suggested by @adamant-pwn

One adjacent question prompted by aug: would there be any interest in supporting augmented matrices directly in the fixed matrix input? For example, an instantiated aug(A,b) model answer could render as one grid with an internal separator, while aug(A,B,C) could produce several blocks. Since aug_matrix currently lives in contrib/matrix.mac, I have not included this in #1847. I am not sure how much demand there would be for this kind of input, but I would be happy to look into it if others would find it useful.

[adamant-pwn]

Just today, I was editing a question and thought "Hm, it would be nice to show this system in row-echelon form", so I think in principle it is quite meaningful to allow such inputs to e.g. check results of forward propagation in Gauss algorithm. Happy to suggest some implementation if there is interest in having this option.

[rmueller-upb]

Thanks for your initiative, I love this idea. When working with Jordan Normal Forms or Frobenius Normal Forms, this also came to my mind.