Fix/usm ndarray suai

[oleksandr-pavlyk]

Closes #583

Sycl USM array interface should use the base pointer in the 'data'
field, rather than its own 'data'.

In [1]: import numpy as np

In [2]: import dpctl, dpctl.tensor as dpt, dpctl.memory as dpm

In [3]: X = dpt.usm_ndarray(10, dtype="|f8")
   ...: Y = np.arange(X.size, dtype=X.dtype)
   ...: X.usm_data.copy_from_host(Y.view("|u1"))

In [4]: print("X =", dpm.as_usm_memory(X).copy_to_host().view(X.dtype))
   ...: print("X[0] =", dpm.as_usm_memory(X[0]).copy_to_host().view(X.dtype))
   ...: print("X[1] =", dpm.as_usm_memory(X[1]).copy_to_host().view(X.dtype))
   ...: print("X[2] =", dpm.as_usm_memory(X[2]).copy_to_host().view(X.dtype))
X = [0. 1. 2. 3. 4. 5. 6. 7. 8. 9.]
X[0] = [0.]
X[1] = [1.]
X[2] = [2.]

Also added a test to cover this usage.

[coveralls]

Coverage increased (+1.0%) to 72.41% when pulling 91e4243 on fix/usm-ndarray-suai into 5f155bc on master.