MATRIX command

Syntax: | `MATRIX m1 { m2 ... } nrow ncol`

This command defines `m1` to be a matrix with `nrow` rows and `ncol` columns, where `nrow > 0` and `ncol > 0` must be scalar values. Other variable names, `m2 ...`, may be included, and they will also be defined to be matrices with the same number of rows and columns.

If `mI` already is a matrix, it will either be trimmed down to the specified dimensions or zero filled to expand it.

If `mI` is an existing variable, but not a matrix, it will be destroyed first.

If `mI` does not exist, it will be created and zero filled.