Fix year-range validation for start/end years (#306, #335)

[brianpm]

Validate user-supplied start_year/end_year against the discovered [found_syear, found_eyear] range instead of membership in a constructed collection, at all four sites (test/baseline x timeseries/history).

• Incorrect year span on plots #306: the timeseries paths used np.arange(found_syear, found_eyear, 1), which is end-exclusive, so a config end_year equal to the last available year was wrongly rejected and silently overridden (shrinking the plotted span by one). Replaced with an inclusive range comparison and removed the now-unused found_yr_range.
• file indexing for files containing more than 1yr of data fails if start or end year are not named in files #335: the history paths tested membership in case_climo_yrs/base_climo_yrs (the set of years parsed from filenames, one per file). When files span more than one year, an in-range year not literally named in a filename was wrongly rejected. Replaced with a range comparison against the found min/max years.

Also corrected the baseline timeseries end-year warning ("first" -> "last"). The warn-and-override fallback is retained; it now fires only for genuinely out-of-range requests.

[nusbaume]

Thanks @brianpm!