make random affine matrix

[rijobro]

create random affine matrix (will be required for inverse transform)
#1515

[wyli]

thanks, need some constraints about the invertibility?

[rijobro]

No, I don't use these for inverting anything, I just use it to write nifti images for the purposes of testing. I need them to have affines along the lines of:

 0, 1,  0, 0
-1, 0,  0, 0
 0, 0, -1, 0
 0, 0,  0, 1

At any rate, this is trivially invertible.