store_data modifies input data structure

[jameswilburlewis]

With this code:

from pytplot import store_data

tp_data={'x':[1,2,3], 'y':[4,5,6]}

store_data('foo',data=tp_data)
store_data('bar',data=tp_data)

the second store_data() call fails because the previous call modified tp_data:

 File "/Users/jwl/PycharmProjects/pyspedas/pyspedas/utilities/tests/tplot_wildcard_tests.py", line 37, in test_tindex_byname
    store_data('bar',data=tp_data)
  File "/Users/jwl/PycharmProjects/PyTplot/pytplot/store_data.py", line 140, in store_data
    values = np.array(data.pop('y'))
KeyError: 'y'

See also: #654

[jameswilburlewis]

The default should probably be to make a deep copy of the input structure before modifying anything. But for performance reasons (time and memory consumption), it might be good to add a "no_copy" option that defaults to False, allowing store_data to skip the copy for the sake of efficiency. I think most usages could probably use no_copy safely....but still, we should default to safety rather than efficiency.