Decide the default matrix binding of the result by majority voting

[yihming]

Issue

Currently, the default matrix binding (i.e. unidata._cur_matrix) is decided by taking the first key from the unidata.matrices map. This way has 2 drawbacks:

1. It has nothing to do with the default matrix bindings of the source data objects. So even if all the sources use key a, it's still possible that the binding of the resulting data object is b.
2. It sometimes could give different binding keys in different executions of the same code. This randomness is definitely not good for the analysis consistency.

Solution

This PR decides it in a more intuitive way: choose the matrix key used in the majority of the source data objects as the new default binding.