editing cells of exprs fails

[gdementen]

e.g. in test_api_larray, if you type "arr3 + 1" and try to edit a value, it fails with:

  File "<ipython-input-11-d6e9ab728323>", line 1
    <expr>[(7, 'F')] = 99
    ^
SyntaxError: invalid syntax

It should either be supported or any expr should be marked readonly automatically.
Supporting this is useless for now (because we cannot save the result anyway) but could be useful in the future when we will be able to create a variable out of the "visible" (filtered) array (#37).