- 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
