added multiple functions to numbers.matrix

added some functions:
- getCol: gets a column from a matrix
- reorderCols: reorders columns in a matrix according to a list of integers
- reorderRows: reorders rows in a matrix according to a list of integers
- reverseCols: reverse the column order
- reverseRows: reverse the row order
- zeros: create a n-by-m matrix of zeros
- zigzag: create a "zigzag" matrix

my reasoning to include zigzag: it's used in Fast Fourier Transforms sometimes and thought it would be neat to have anyway.