Add trading_index function

[maread99]

WIP. DO NOT MERGE!
To be merged only after all calendars moved to new test suite (as described on #86) and TODO noted further below have been completed.

Adds trading_index function to allow for creating a trading index over a given range of sessions at a given period. Similar to pandas_market_calendars.date_range with additional functionality:

• index can be created as a pd.DatetimeIndex of indices or pd.IntervalIndex of intervals.
• provides for independent force_break_close and force_close options
• takes advantage of exchange_calendars use of nanos for speed improvement (evaluates trading index for XHKG (a calendar with breaks) over a year at 30 minute periods in <2ms).

Credit to @Stryder-Git at pandas_market_calendars for showing the way with a vectorised implementation.

TODO post resolution of #71:

• Move trading_index function to method of ExchangeCalendar (leave underlying _TradingIndex on calendar_helpers).
• Tests:
  • include hypothesis fuzz tests - mark so they do not run by default (amend CI test parameters to recognise mark). Add hypothesis to dev requirements.
  • include test with known input / output to default test suite (employ Answers to be provided by side option to define minute bounds / test suite overhaul #71).
• Add example to calendar_methods.ipynb.

[maread99]

Superseded by #100.