support single precision in state vector simulator

[hhorii]

Summary

This PR adds a new parameter precision for qasm simulator.

Details and comments

QubitVector<std::complex<double>*> is changed to QubiveVector<double | float>.
By using single-precision, simulation becomes faster with reducing precision.

[chriseclectic]

@hhorii @atilag when using single precision do we have to worry about narrowing of data types when applying matrices given in double precision? The lambda loops will be things like data_[i] = mat(i, j) * data_[j] where mat(i,j) is complex<double> and data_[j] is complex<float>. Do we need to static cast the matrices to the reduced precision first?

[atilag]

This is valid narrowing conversion between two floating point types, it's stated in the standard as implicit so there's no need for static_cast. Having said that, I really hate implicit behaviors, it's great for programming faster but in the long term are less readable so maintainable. If there's not many conversions I'd explicitly use static_cast<float> and make clear that this is the behavior we want.
There's always a round in the last decimal digit though, so If this is mathematically correct for our use-case, then it's fine.

[chriseclectic]

@hhorii I'm ready to merge this PR, but it needs to be rebased from master to get the build to work. The merge update failed due to some changes in recently merged PRs (I think in the UnitaryMatrix class).