New Part BASIC LINEAR ALGEBRA: Splitting section for matrices and determinants
New Part BASIC LINEAR ALGEBRA: Splitting section for matrices and determinants

Restructuring as discussed in #769 continued:
- The section "Matrices and determinants" are split into two sections "Matrices" and "Determinants"
- additional theorems in AV's Mathbox

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