Add heading to bird tracking data

[Peter Desmet]

There is a field direction in the tracking data (see https://github.com/inbo/bird-tracking-etl/blob/master/vignettes/bird-tracking-etl.Rmd#output-format) with the definition:

Heading in degrees (0 is north, 90 is east).

Probably good to add that in dynamicProperties.

[Peter Desmet]

Now added in dynamicProperties as:

\"heading_in_degrees\":", str_replace_na(sprintf("%.3f", round(direction, digits = 3)), "null")

The values range from -180 to 180 and are rounded to 3 digits with trailing 0. NA values are shown as the JSON valid value null (also updated for second_since_last_occurrence in samplingProtocol).

Will be included in next wmh / gull data publication