Added warning for ADS.clear()

[SilkeSchomann]

Description of work

Calling clear() for the ADS now displays a warning.

Occasionally, users call clear() for the ADS in order to get rid of temporary workspaces. This can cause problems for some of our interfaces, as there might be hidden workspaces that are still in use by the interface. One example of this is MSlice: mantidproject/mslice#839

There is no associated issue.

To test:

Run the following script and check that the warning makes sense:

from mantid.simpleapi import *
import matplotlib.pyplot as plt
import numpy as np

from mantid import mtd

mtd.clear()

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[jhaigh0]

Works as intended :) Maybe it'd be nice to add an optional argument to clear() to make it output what workspaces its clearing (including hidden ones). Might help with debugging these kinds of problems