Allow for 'halo_radius' to be set as float value

[springmeyer]

Currently the Text Symbolizer accepts halo_radius as an unsigned type, but in cases where small text is rendered the halo can appear too thick relative to the text size. It would be great to be able to set this value as a float, to allow for smaller halo size.

If a user attempts to use a float value an informative python traceback is given:

Traceback (most recent call last):

File "world_population.py", line 7, in <module>

load_map(m, mapfile)

UserWarning?: Failed to parse attribute 'halo_radius'. Expected type unsigned but got '.5' in TextSymbolizer in style 'countries_label'