added roi, noise and gain (#143)

- Moved definitions of Cluster_2x2 and Cluster_3x3 to it's own file
- Added optional members for ROI, noise_map and gain_map in ClusterFile

**API:**

After creating the ClusterFile the user can set one or all of: roi,
noise_map, gain_map

```python
f = ClusterFile(fname)
f.set_roi(roi) #aare.ROI
f.set_noise_map(noise_map) #numpy array
f.set_gain_map(gain_map) #numpy array
```

**When reading clusters they are evaluated in the order:**

1. If ROI is enabled check that the cluster is within the ROI
1. If noise_map is enabled check that the cluster meets one of the
conditions
    - Center pixel above noise
    - Highest 2x2 sum above 2x noise
    - 3x3 sum above 3x noise
1. If gain_map is set apply the gain map before returning the clusters
(not used for noise cut)

**Open questions:**
1. Check for out of bounds access in noise and gain map?

closes #139 
closes #135 
closes #90

tests/test_config.hpp.in

@@ -7,6 +7,6 @@ inline auto test_data_path(){
     if(const char* env_p = std::getenv("AARE_TEST_DATA")){
         return std::filesystem::path(env_p);
     }else{
-        throw std::runtime_error("AARE_TEST_DATA_PATH not set");
+        throw std::runtime_error("Path to test data: $AARE_TEST_DATA not set");
     }
 }