Now integrate() can return the full trajectory and plotted in the Python; improved readablity

loadtxt.h

@@ -0,0 +1,14 @@
+#pragma once
+#include <string>
+#include <vector>
+class Loadtxt {
+public:
+    Loadtxt(std::string file_name, std::vector<int> cols);
+    ~Loadtxt();
+    std::vector<std::vector<double>> get_cols();
+private:
+    const char *whitespaces = " \t";
+    void line_to_buf(std::vector<int> cols, std::string line, double *buffer);
+    double *buffer;
+    int n_rows, n_cols;
+};