Naming mismatch for some target group operations

[mmaker]

Currently msm_Gt is meant for multipairing of the form $\sum_i e(A_i, B_i)$ where $A_i \in \mathbb{G}_1$ and $B_i \in \mathbb{G}_2$. We should support also $\sum_i a_i B_i$ where $a_i \in \mathbb{F}_r$ and $B_i \in \mathbb{G}_t$