read_habitatsprings(): provide argument 'units_7220'

[florisvdh]

An aggregated points object can now be returned (FYI @Patrikoosterlynck):

units_7220 If TRUE, an sf object of type-7220-locations is
returned at the population unit level.
To accomplish this, the data source is aggregated by unit_id.
Multiple points belonging to the same unit are replaced by their
centroid, their area attribute is summed (if all values are known)
and for other attributes the maximum value is returned.

With units_7220 = TRUE, variable point_id is dropped and
an extra attribute variable nr_of_points is added.
It represents the number of points that belong to each unit.

This will be merged in the release candidate for version 0.2.0 (branch rc0.2)