`wdmexp`: helper function to export data

[rburghol]

Examples:

• Export river EOS values for DSN 11 from scenario subsheds:
  • cbp wdmexp subsheds river/eos JL1_6560_6440.wdm 11 1984 2020
• Export river inflow from upstream and EOS combined -- DSN 11 river
  • cbp wdmexp subsheds river/stream JL1_6560_6440.wdm 11 1984 2020
• Export data for met WDMs, DSN 1000, scenario subsheds, full time period through 2022
  • cbp wdmexp nldas1221 met met_N51165.wdm 1000 1984 2022
• Export/Summarize DSN 111 flow Out of a given segment
  • cbp wdmexp p6ss river/stream JL1_6560_6440.wdm 111 1984 2020