Reformat start & end year values

[terryf82]

There are several different interpretations of the _startyear and _endyear values provided in the city-specific configs:

• in some situations, end_year is treated as 01-01-end_year and data is filtered out if it it occurred in the same year as end_year (that is, if end_year is 2018 and a crash happened in 2018, the crash is filtered)

• in other situations, end_year is treated as 31-12-end_year and data is filtered out only if it occurred in a year after end_year (so if end_year is 2018 crashes in 2018 are included, crashes in 2019 are excluded).

We need to review all uses of these values and align under a common understanding. It's probably worth swapping to full start_date and end_date values that include day, month and year to clear up any confusion.

[terryf82]

Completed as part of PR https://github.com/Data4Democracy/crash-model/pull/211