Given an object with class `spacetime`

, determine if it is a *spacetime cube*.
If the time-series is is irregular a warning is emitted
(see `validate_spacetime()`

for more on the restrictions on the time column.

## Details

A spacetime object is a spacetime cube when it contains a regular time-series representation of each geometry. That is, only one observation for at each time period per geography is present.

The number of rows in a spacetime cube is the number of geographies multiplied by the number of time periods. For example if there are 10 locations and 20 time periods, the number of rows must be 200.

## Validation

`is_spacetime_cube()`

runs a number of checks that to ensure that the provided
object is in fact a spacetime cube. It checks that:

the number of rows is equal to the number of locations multiplied by the number of time periods

each time period has an equal number of observations

each location has an equal number of observations

each combination of time period and location has only one observation

that the time-series is regular

## Examples

```
df_fp <- system.file("extdata", "bos-ecometric.csv", package = "sfdep")
geo_fp <- system.file("extdata", "bos-ecometric.geojson", package = "sfdep")
# read in data
df <- readr::read_csv(df_fp, col_types = "ccidD")
geo <- sf::read_sf(geo_fp)
# Create spacetime object called `bos`
bos <- spacetime(df, geo,
.loc_col = ".region_id",
.time_col = "time_period")
is_spacetime_cube(bos)
#> [1] TRUE
is_spacetime_cube(bos[round(runif(1000, 0, nrow(bos))),])
#> ! Number of rows does not equal `n time-periods x n locations`
#> [1] FALSE
is_spacetime_cube(guerry)
#> [1] FALSE
```