Add "pfaffian" method

[d-torrance]

We already have pfaffians for computing an ideal containing all the sub-Pfaffians of a particular size, but we may just want the Pfaffian of a skew-symmetric matrix as a number/polynomial (similar to determinant v. minors).

i1 : R = QQ[a..f];

i2 : genericSkewMatrix(R, 4)

o2 = | 0  a  b  c |
     | -a 0  d  e |
     | -b -d 0  f |
     | -c -e -f 0 |

             4      4
o2 : Matrix R  <-- R

i3 : pfaffian oo

o3 = c*d - b*e + a*f

o3 : R