Adjust TensorflowVector square and square root computations.

For some reason, `tf.square(x)` and `tf.pow(x, 2)` can result in
distinct values for small-enough values of x. The same is true of
`tf.sqrt(x)` and `tf.pow(x, 0.5)`.

This commit ensures the `tf.sqrt` and `tf.square` functions are
used in these two edge cases for the `vector ** p` syntax. This
choice is motivated by the fact that these functions are used
in most official tensorflow / keras code, and seem to be closer
to their torch counterparts (which are equivalent to `vec ** p`)
than `tf.pow` - based on a few local tests.