Update reflectivity ppi file names and directory structure

[peterdesmet]

File name is composed of:

txt = to indicate it is ascii
BE = Belgium
61 = Zaventem
VOL = ?
NA = ?
201304052202 = datetime

@leijnse can you figure out if this complies with OPERA file name practice and if we should rename files (e.g. drop txt_ at beginning).

[peterdesmet]

Proposed new directory and file structure (verify with OPERA file name convention, e.g. drop PPI_?

/ppi
- 6260_20130405.zip
  - 6260_201304052202.txt
  - 6260_201304052207.txt
- 6260_20130406.zip
- ...
/position
- 6260_lat.txt
- 6260_lon.txt
- ...

[peterdesmet]

Position data added in 6fd7ae3

[leijnse]

There is no official OPERA file naming convention. I've adopted the naming conventions suggested above. Files have been uploaded to Google Drive.

[peterdesmet]

Reflectivity data has been uploaded: https://github.com/enram/case-study/tree/master/data/reflectivity-ppi