Set threads warnings

[stavros11]

Fixes #442 by:

• Modifying qibo.set_threads to update the number threads only for the currently active backend.
• Raising warnings when using qibo.set_threads with the numpy or tensorflow backends which do not support this kind of thread setting.

[codecov]

Codecov Report

Merging #443 (9ad5edf) into master (47ad4ad) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #443   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           83        83           
  Lines        11866     11883   +17     
=========================================
+ Hits         11866     11883   +17     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/backends/__init__.py	100.00% <100.00%> (ø)
src/qibo/backends/numpy.py	100.00% <100.00%> (ø)
src/qibo/backends/tensorflow.py	100.00% <100.00%> (ø)
src/qibo/tests/test_backends_init.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 47ad4ad...9ad5edf. Read the comment docs.

[scarrazza]

Suggested change

	"Cannot change the number of threads.")
	" Cannot change the number of threads.")

[stavros11]

Fixed, thanks.

[scarrazza]

Great, thanks. Could you please fix another issues I have just observed, when changing to numpy the device list is [], for symmetry with other backends I think we could simply say something like /CPU:0...

[stavros11]

Great, thanks. Could you please fix another issues I have just observed, when changing to numpy the device list is [], for symmetry with other backends I think we could simply say something like /CPU:0...

I changed the default numpy device to "/CPU:0". Let me know if you agree with the last changes.

[scarrazza]

Perfect, thank you.