Allow PAD operator to pad with non-zero

PAD now takes an additional attribute, with the padding value.
Will generally be zero, but other values are allowed.

tensor_read now requires the coordinates to be within the
given tensor, with unpredictable behavior occurring if an access
outside of the tensor occurs. Callers of tensor_read are expected
to check the coordinates and take the appropriate action. The
primary impact of this is to move the responsibility for padding to
each operator. In practice, this is expected to not be a functional
change, but a cleanup to make the behavior more clear.

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: I4f21ca9a13d82d422bbd66c400f23aa9a0bd2aa0