Add option for InputParser to not need all lists defined in the input file

If a key is not in the file just return and empty vector

src/inputs/InputParser.hpp

@@ -118,6 +118,9 @@ template <typename T>
 std::vector<T> as_vector(pt::ptree const &pt, pt::ptree::key_type const &key)
 {
     std::vector<T> r;
+    if(pt.count(key) == 0)
+        return r;
+
     for (auto& item : pt.get_child(key))
         r.push_back(item.second.get_value<T>());
     return r;

tests/exec_test/default/sisso.json

@@ -10,6 +10,5 @@
     "n_models_store": 1,
     "leave_out_inds": [0, 1, 2, 60, 61],
     "opset": ["add", "sub", "abs_diff", "mult", "div", "inv", "abs", "exp", "log", "sin", "cos", "sq", "cb", "six_pow", "sqrt", "cbrt", "neg_exp"],
-    "param_opset": [],
     "fix_intercept": false
 }