Create glossary

[cczhu]

As part of documenting TEPs, we need to create a glossary of terms, probably on bdit_traffic_prophet's Readthedocs. For now, here are some terms to add:

• AADT - annual average daily traffic
• MADT - monthly average daily traffic
• DoMADT - day-of-week over month average daily traffic, used in countmatch to determine goodness of fit between permanent and short-term traffic counts.
• permanent count, permanent traffic count (PTC) - traffic count data from permanent or semi-permanent stations. A permanent station must have data for all months and at least 75% of all days of the year entirely covered by data to be considered a permanent station for that year. PTC is used in Bagheri et al., but not commonly used here to avoid confusion with private transportation companies.
• short-term count, short-term traffic count (STTC) - traffic count data from locations without a permanent or semi-permanent station.