Fix a bunch of warnings in C++.

- returning reference to temporary in MatDiag::operator()()
- delete instead of delete[] after a new[] in mx2Faust.hpp
- src/faust_linear_operator/CPU/faust_linear_algebra.hpp warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
- src/faust_linear_operator/CPU/faust_MatDense.hpp:1135:2: warning: 'delete' applied to a pointer that was allocated with 'new[]'; did you mean 'delete[]'? [-Wmismatched-new-delete]
- src/faust_linear_operator/CPU/faust_MatDense.hpp:1302:15: warning: '&&' within '||' [-Wlogical-op-parentheses]