Feature: Merged kernel-fusion branch

* Fuses multiple AXPY kernel
* Possibility to add thread-wise for loops in AXPY-like kernels

examples/tie.cpp

@@ -0,0 +1,17 @@
+#include "isaac/array.h"
+#include "isaac/symbolic/execute.h"
+#include "isaac/symbolic/io.h"
+
+namespace sc = isaac;
+
+int main()
+{
+//    sc::array A(10,10);
+//    sc::array U(10, 10);
+//    sc::array V(10, 10);
+//    sc::array x(10), y(10);
+////    float c = .2, s = .4;
+//    sc::math_expression tree = sc::sfor(0, 10, 1, sc::rot(sc::col(U, sc::_i0), sc::col(U, sc::_i0 + 1), x[sc::_i0], y[sc::_i0]));
+//    std::cout << to_string(tree) << std::endl;
+//    sc::execute(tree);
+}