add Shashlik to CaloTowers

This commit adds the Shashlik ECAL to the CaloTowers algorithm. It treats the subdet ID EcalShashlik as equivalent to EcalEndcap, passes the EcalRecHitsEK input tag to the algorithm, and specifies some preliminary thresholds equal to 1/2 of the current EB thresholds (based on a suggestion from Shashlik experts).

Validation of the changes, using runTheMatrix 13815 (QCD_Pt_600_800_8TeV with Extended2023SHCalNoTaper), plotting average CaloTower EM energy vs. eta:
old: 
![caloem_old](https://cloud.githubusercontent.com/assets/4672808/3627559/f7982be8-0e8a-11e4-90f9-4dd6f472ffa4.png)

new:
![caloem_new_th](https://cloud.githubusercontent.com/assets/4672808/3627563/fcd17cf4-0e8a-11e4-8922-1595b32234ca.png)