Minimise rounding error for `Real` input to `Periodic`

For `TimeType` data, `Periodic` transform [uses the `_periodic` function](https://github.com/invenia/Transforms.jl/blob/2e23f97516ce6a032d66ce20d297406640cf49aa/src/periodic.jl#L50) to compute the periodic function for each instant in time.

The purpose of the `_periodic` function is to find an "origin" in time that minimises rounding error. This is particularly important when working with dates, because the calculations are done on milliseconds, and the number of milliseconds since "year 0" is enormous.

```
julia> sin(2π)
-2.4492935982947064e-16

julia> sin(2π*100)
3.928773447456944e-15

julia> x = Millisecond(DateTime(2021, 02, 09) - DateTime(0)).value
63780048000000

julia> sin(2π*x)
-0.020117088546809787
```

So `_periodic` finds the most recent time, `period_begin`, such that `period_begin / period` is a whole number, and in turn `sin(2π * period_begin / period) ≈ 0` with the highest precision.

But this consideration could just as well apply to `Real` numbers, albeit less often in practice. So we should accomodate `Real` input in `_periodic` or define a new equivalent method, e.g.

```
function _periodic(f, instant, period, phase_shift=0)
return f(2π * (mod(instant, period) - phase_shift) / period)
end
```

Examples:

```julia
julia> _periodic(sin, 7/4, 7.)
1.0

julia> _periodic(sin, 0, 7.)
0.0

julia> _periodic(sin, 7e14, 7.)
0.0

julia> _periodic(sin, 7e14 + 7/4, 7.)
1.0
```