Fix use of deprecated numpy fromstring functionality

[k-collie]

Description of work

• Remove unused import
• Use numpy.frombuffer rather than numpy.fromstring as the latter is deprecated

Fixes

• Use of deprecated numpy.fromstring, see deprecation note under sep in docs

Reproduce deprecation note

Note deprecation warning is hidden by sal, can be seen by installing numpy>=1.14 and running:

import numpy as np
np.fromstring(b'abc', dtype=np.int8)

To test

• Test getting a signal from a concrete SAL server.
• Note that unit tests do not cover _decode_array so running them will not test this change.

Reviewer checklist

• The author suggested tests are successful.
• Relevant documentation (user and developer) has been added, and is clear and complete.
• Suitable unit tests have been added. If unit tests are not included in this MR, please comment as to why and create an issue to address this.