FILLVALUE function fills gaps in a bucketized Geo Time Series™ instance with a fixed value/location/elevation.
It expects a GTS or a list thereof on the top of the stack after having consumed a list of parameters (latitude, longitude, elevation, value) sitting on top of the stack prior to the call.
Location must be floating point numbers (e.g. 0.0 not 0). Elevation must be an integer. If no location or elevation should be set, NaN can be used instead of specific value.
This function has no effect on non bucketized GTS instances.
The value is cast to the type of the GTS value