SYNOP precip code definitions

In the table below, "ind3" means a report that indicates "no precip",
presumably since the last regularly scheduled report.

+----+----------------------------------------------------------+
| code | description                                            |
+------+--------------------------------------------------------+
|  0   | 6h direct from 6h precip reports                       |
|  1   | 6h from two 3h accumulations                           |
|  2   | 6h from a 3h accumulation preceded by a 3h ind3 report |
|  3   | 6h from a 3h ind3 report preceded by a 3h accumulation |
|  4   | a 6h zero from two 3h ind3 reports                     |
|  7   | a 6h zero from a 6h ind3 report                        |
|  9   | a 6h zero from a 9h zero report                        |
| 10   | a 6h zero from a 9h ind3 report                        |
| 12   | a 6h zero from a 12h zero report                       |
| 13   | a 6h zero from a 12h ind3 report                       |
| 18   | a 6h zero from an 18h zero report                      |
| 19   | a 6h zero from an 18h ind3 report                      |
| 24   | a 6h zero from a 24h zero report                       |
| 25   | a 6h zero from a 24h ind3 report                       |
+----+----------------------------------------------------------+