Fix incomplete Issue #110 - change PREMode default to NOT_SET

[BurakKTopal]

This PR replaces this original PR and fixes #110

The original fix in commit 64ed308 never worked because PREMode defaulted to INDCPA, not NOT_SET. This caused GetParamsPK() to always return Q×P basis for HYBRID, even when PRE is not needed.

Changes:

• Set default PREMode to NOT_SET in gen-cryptocontext-params-defaults.h
• Update pre-buffer.cpp to explicitly set PREMode=INDCPA

[dsuponitskiy]

Yuriy asked me to fix your original PR, so please do not reopen this one. We don’t need duplicate PRs. In addition, I’ve already started two extensive CI runs for the PR I opened.

[BurakKTopal]

@dsuponitskiy and @yspolyakov, I understand and respect the policy. If I may comment that I find the steps taken unusual: taking over someone else's work like this. I would have valued it a lot more to let me do it end-to-end and obtain full credits.