issue-204: period::intersects incorrect for empty periods

[jeking3]

Originally identified by Coverity Scan. Zero length periods like [3, 3) cannot intersect with anything. Invalid periods like [5, 4) also cannot intersect with anything. In both cases the unit tests are wrong, and fixing them reveals the implementation is not canonical for half-open sane intervals.

In testgeneric_period.cpp there is a bug on lines 128-129; again on 149-150:

  check("Intersects", zero_len.intersects(p1));
  check("Intersects", p1.intersects(zero_len));
...
  check("Intersects", null_per.intersects(p1));
  check("Intersects", p1.intersects(null_per));

Empty (null) periods cannot intersect with anything, so the test is wrong. Fixing the test reveals the implementation is wrong.

On simplifying the implementation of intersects to use the half-open canonical check (ensuring the half-open intervals are sane), it now passes.

  template<class point_rep, class duration_rep>
  inline BOOST_CXX14_CONSTEXPR
  bool period<point_rep,duration_rep>::intersects(const period<point_rep,duration_rep>& other) const
  { 
    return !is_null() && !other.is_null() && (begin_ < other.end()) && (other.begin_ < end());
  }
...
  check("Intersects", !zero_len.intersects(p1));
  check("Intersects", !p1.intersects(zero_len));
...
  check("Intersects", !null_per.intersects(p1));
  check("Intersects", !p1.intersects(null_per));
  ***passed***

This fixes #204

[JeffGarland]

This looks correct to me

[JeffGarland]

The CI issues seem spurious.