Update ExchangeCalendar.trading_index

Fixes bug that included close (wrongly) when `force` was True,
`intervals` was False and closed was `left` or `neither`. Revises test.

Revises so that `start` and `end` can be passed as either date or
minute. Adds `TestTradingIndex.test_start_end_times`.

Updates tutorial `trading_index.ipynb`.

Also:
- adds following functions to `calendar_helpers` and associated
test(s) to `test_calendar_helpers`:
  - `is_date`
  - `to_utc`
  - `parse_date_or_minute`