submatrix specification

[DanGrayson]

Make specification of submatrices involving long intervals of consecutive indices more efficient:

i1 : M = ZZ^6

       6
o1 = ZZ

o1 : ZZ-module, free

i4 : M_{1,[3,5]}
stdio:4:2:(3): error: expected list of integers

i5 : M_{1,3,4,5}

o5 = | 0 0 0 0 |
     | 1 0 0 0 |
     | 0 0 0 0 |
     | 0 1 0 0 |
     | 0 0 1 0 |
     | 0 0 0 1 |

              6        4
o5 : Matrix ZZ  <--- ZZ

[d-torrance]

M_{1,3..5} works. Is this still an issue?