't()' function for calculating GEV quantities

`gev_t_function(x, location, scale, shape)`

## Arguments

- x
Argument of the function; vectorized.

- location
Location parameter; numeric.

- scale
Scale parameter; positive numeric.

- shape
Shape parameter; numeric.

## Note

The shape parameter is not vectorized. This function is only
intended to be used when location, scale, and shape are scalars.

## See also

See the Wikipedia entry for the GEV distribution,
https://en.wikipedia.org/wiki/Generalized_extreme_value_distribution