Attenna height for the Netherlands

[bart1]

Building checks to ensure meta data is correct (#143) i noticed the height for the Netherlands does not correspond between the opera meta data and the polar volumes.

The differences are minute (a few meters):

radar	opera	polar volume
nlhrw	25	27.7
nldhl	55	51

I guess this comes from the values being hard coded here:

getRad/inst/extra/KNMI_vol_h5_to_ODIM_h5.c

Lines 71 to 73 in 156b846

	height_DeBilt = 44;
	height_DenHelder = 51;
	height_Herwijnen = 27.7;

@leijnse do you know what is correct? Then I can fix it