interpolate db inputs for interim years

[bricegnichols]

Most inputs are only available for standard forecast years (2018, 2030, 2040, 2050), but users want to run interim years like 2044. The main impediments to this are some tables in the input database. If a year isn't available from this database we should interpolate between the data we have (e.g. for parking rates, which are recomputed for future years).