For the operations that we haven't implemented directly in ScVectorRefMatrix
yet, create the ScFullMatrix the same way as we used to do before, and call
the appropriate method of ScFullMatrix.

This commit only moves the code, does not turn the 'throw
std::runtime_exception' into actually using the new ensureFullMatrix().

Change-Id: I336a4ddf07b0b60a5a41036e19c70a44ba575834