Moving subsections about matrices to main set.mm
Moving subsections about matrices to main set.mm

* section "The subalgebras of diagonal and scalar matrices" moved from AV's mathbox to main set.mm
*  section "Matrices of dimension 0 and 1" moved from AV's mathbox to main set.mm
* some auxiliary theorems (many of them about matrices) moved from AV's mathbox to main set.mm

metamath/set.mm,metamath/set.mm,metamath/set.mm