make /how attribute of scans in read.pvol() optional

[plieper]

It looks like quite some countries (NL, BE, FR for example) do not have a subfolder how under dataset1/2/3/.... However, the function read.pvol() requires this and trying to read in pvols that do not have the how subfolder makes the function crash with the error: Error in h5readAttributes(filename, paste(scan, "/how", sep = "")) : Object dataset1/how does not exist in this HDF5 file.

One way to solve this would be to make the how attribute optional in the function read.pvol().

[adokter]

This error message has been solved already a while ago.

[adokter]

commit a58a3fb now also checks the presence of what, where and how groups at the root.