add terms to glossary

[davelandry]

• [x] Headcount Ratio
• [x] Poverty Gap Ratio
• [x] Poverty Severity Ratio
• [x] Rainfall Variability
• [x] explanation of "age of reference"

[davelandry]

@jspeis what about age of reference?

[jspeis]

we can simply alter the term "age of reference" to make it more straighforward e.g.

Wasted: Below minus two standard deviations from median weight for height of the given population.