ktp.next() for keys

[m-temp]

As discussed in https://github.com/lowRISC/ot-sca/pull/115 and https://docs.google.com/document/d/1sZjASiUji_IT-t9jEFlK-kGc_Gw3mGfubNvpkM_IQT4/edit#heading=h.qd65x0y76b0e we should check the usage of key, text = ktp.next() in cw/capture.py

Default behavior is that ktp.next() returns const_key, random_plaintext. This default behavior can be changed with ctx.obj.ktp.fixed_key = False.

Afaik, we call ktp.next()[0] only once per function and use only the first call of the iterator to assign a constant key. For random keys we use ktp.next()[1]. Thus, I think we ware fine and can change the default behavior, but we should double check that and read the spec if the first call of the iterator still outputs a deterministic value. On the other hand, if we really rely on a constant non random value, we should use a constant and not a constant iterator.