Paice's method of evaluating stemming algorithms.

Paice's method of evaluating stemming algorithms by calculating understemming and overstemming indexes, stemming weight, and error-rate relative to truncation (ERRT).

Based on Paice: An evaluation method for stemming algorithms.
Please provide a full citation such as:
```
Chris D. Paice (1994). An evaluation method for stemming algorithms. In Proceedings of SIGIR, 42--50.
```