Target.md

+What can this do ?
+-------------
+In AUSAlib there is support for a [Target](https://gitlab.au.dk/ausa/ausalib/blob/master/include/ausa/setup/Target.h) which is a collection of [Layers](https://gitlab.au.dk/ausa/ausalib/blob/master/include/ausa/eloss/Layer.h).  
+A Layer consists of a [Material](https://gitlab.au.dk/ausa/ausalib/blob/master/include/ausa/eloss/Material.h), a geometrical shape called a [Volume](https://gitlab.au.dk/ausa/ausalib/blob/master/include/ausa/geometry/Volume.h).
+
+With a Target one can ask about which layers the line between points intersect. The signature of this is
+```c++
+std::vector<Intersection> getIntersections(const TVector3& from, const TVector3& to);
+```
+where the number of entries in the returned vector corresponds to the number of intersections ordered by distance to the intersection point.  
+
+Each intersection contains the following information:
+```c+++
+struct Intersection {
+   double distance;    // Distance to intersection
+   double transversed; // Distance particle will transverse in layer.
+   const Layer* layer; // What layer it hit. You can get the material from this.
+   size_t index;       // Index of that layer, if you have created a number of EnergyLossCalculators
+};
+```
+
+JSON
+-------------
+
+The easiest way to create a Target is to create it from json:
+
+```json
+{
+  "center" : {"x":"5mm", "y":"5mm", "z":"5mm"},
+  "orientation" : {"x":"0mm", "y":"1mm", "z":"0mm"},
+  "normal" : {"x":"0mm", "y":"0mm", "z":"-1mm"},
+  "transverseLength" : "50mm",
+  "upLength" : "50mm",
+
+  "layers" : [
+    {"material" : "Wolfram", "thickness" : "50nm"},
+    {"material" : "Compound", "thickness" : "50nm"},
+    {"material" : "Silicon", "thickness" : "50nm", "active" : true}
+  ],
+
+  "materials" : [
+   {"name":"Compound", "density": "19.25g/cm^2", "Z":[74, 84], "mass" : ["100 g/mol", "200 g/mol"], "stoichio" : [5, 6]},
+   {"name":"Wolfram", "density": "19.25g/cm^2", "Z":74, "mass" : "183.84 g/mol"}
+  ]
+}
+```  
+For details about materials have a look [here](https://gitlab.au.dk/ausa/ausalib/wikis/materials).
+
+The target layers created from this will be boxes with dimensions (upLength, transverseLength, thickness).
+
+Basic usage 
+-------------------------
+This shows how to read in a Target from json and query it for intersections:
+```c++
+#include <ausa/json/IO.h>
+
+using namespace AUSA;
+using namespace std;
+
+int main(int argc, char* argv[]) {
+    Target target = JSON::readTargetFromJSON("target.json");
+                                                 /* From */  /* To */
+    auto intersections = target.getIntersections({5,5,-100}, {5,5,5});
+
+    for (auto& i : intersections) {
+        cout << "Hit layer " << i.layer->getMaterial().getName() << " and moves " << i.transversed << " mm in it" << endl;
+    }
+}
+```
+
+Using the above target json file this will output:
+```
+Hit layer Wolfram and moves 5e-05 mm in it
+Hit layer Compound and moves 2.5e-05 mm in it
+```
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