Add integral function for time_weight

[syvb]

Implements functionality requested in #455:

• Adds an experimental integral(tws[, unit]) -> float8 function (unit defaults to 'second')
• Adds an experimental interpolated_integral(tws, start, interval, prev, next[, unit]) -> float8 function (unit defaults to 'second')
• Adds experimental arrow functions for integral/interpolated_integral
• Makes trapezoidal an alias for linear in the time_weight function

The unit parameter to integral/interpolated_integral is a string that specifies what time unit to use for the returned f64. It can be any fixed-duration unit that PostgreSQL allows in a interval. An alternative would be to have an interval be passed as the unit instead, but intervals can have variable length units (days and months) which might not be wanted. Also, integral(..., 'hour') is clearer than integral(..., '1 hour'::interval).

[syvb]

I removed the trailing whitespace here for consistency with the similar SQL for getting the integrals.

[rtwalker]

Thanks for the changes! Looks good to me though I'd wait for someone else to weigh in :)

[davidkohn88]

LGTM!

[syvb]

bors r+

[bors]

Build succeeded:

• Clippy/rustfmt Test
• Test Postgres (12)
• Test Postgres (13)
• Test Postgres (14)