matrix(MutableMatrix) -- make a matrix from a mutable one

Synopsis

Usage:
matrix m
Function: matrix
Inputs:
- m, an object of class MutableMatrix
Outputs:
- a matrix, the same matrix as m, but not mutable.
Optional inputs:
- Degree => ..., -- not really an option?
- DegreeMap => ..., -- not really an option

There are many more operations available for matrices than for immutable matrices, but mutable matrices are sometimes easier to construct.

i1 : m = mutableZero(ZZ,3,5)

o1 = 0

o1 : MutableMatrix

i2 : m_(1,2) = 3

o2 = 3

i3 : m_(2,4) = 54

o3 = 54

i4 : m_(0,0) = -12

o4 = -12

i5 : m

o5 = | -12 . . . .  |
     | .   . 3 . .  |
     | .   . . . 54 |

o5 : MutableMatrix

i6 : matrix m

o6 = | -12 0 0 0 0  |
     | 0   0 3 0 0  |
     | 0   0 0 0 54 |

              3        5
o6 : Matrix ZZ  <--- ZZ