Add ability to write HYDAT flags to time series

[analytophile]

add functionality to rvn_rvt_tidyhydat() to write a time series of HYDAT data flags in the following format:

:ObservedData HYDROGRAPH_FLAGS 228009537
1991-06-01 00:00:00 1 11172
0 # default - no flag
1 # partial (A)
2 # ice (B)
3 # dry(D)
4 # estimate (E)
...
0
0
3
0
:EndObservedData

This info should be already accessible via tidyhydat()