make as.data.frame.vpts the inverse of as.vpts

[adokter]

Add the ability to change a vpts object into a data.frame, and then cast it back into a vpts, as in:

as.vpts(as.data.frame(example_vpts))

Current implementation fails because as.data.frame returns several columns that have a different naming convention than VPTS CSV standard. Suggest changing the output data.frame column names as follows:

lon -> radar_longitude
lat -> radar_longitude
antenna_height -> radar_height

Also a column radar_wavelength needs to be added

[adokter]

closed by 33001b0

[iskandari]

test added in 708e2e4