Allow multiple heatmaps to be displayed simultaneously.
Following screenshots show energy for Receiver 2, then Receivier 3, then both receivers:
![rx2](https://user-images.githubusercontent.com/1108513/29282036-37446622-8119-11e7-84c2-fe21c29460b5.PNG)
![rx3](https://user-images.githubusercontent.com/1108513/29282040-38fa4ef0-8119-11e7-9735-fe2792337b48.PNG)
![rx_both](https://user-images.githubusercontent.com/1108513/29282043-3c0e3d22-8119-11e7-8ac1-67694dd0dc48.PNG)
Note: the table on the left has Show Count clicked, so it shows the number of measurements in each cell, useful for determining sensor coverage.