[pybullet] Add calculateJacobian.

* Add the calculateJacobian method to the pybullet API.
* Adjust the shared memory interface to handle fixed/floating bases
  in the calculateJacobian method.
* Fix a few comments.

src/BulletInverseDynamics/IDMath.cpp

@@ -69,7 +69,7 @@ idScalar maxAbsMat3x(const mat3x &m) {
 
 void mul(const mat33 &a, const mat3x &b, mat3x *result) {
     if (b.cols() != result->cols()) {
-        error_message("size missmatch. a.cols()= %d, b.cols()= %d\n",
+        error_message("size missmatch. b.cols()= %d, result->cols()= %d\n",
                       static_cast<int>(b.cols()), static_cast<int>(result->cols()));
         abort();
     }