Please expose the GetUnits() on the FrAdcData

Please expose the GetUnits call on the FrAdcData object. Something like this would probably work fine.

--- ldas-tools-framecpp-swig-2.6.4/common/FrAdcData.i	2018-08-27 13:23:28.000000000 -0700
+++ ldas-tools-framecpp-swig-2.6.4.working/common/FrAdcData.i	2018-10-02 18:09:34.295248056 -0700
@@ -106,7 +106,7 @@
   nBits_type GetNBits( ) const;
   bias_type GetBias( ) const;
   slope_type GetSlope( ) const;
-  // STRING GetUnits( ) const;
+  std::string GetUnits( ) const;
   sampleRate_type GetSampleRate( ) const;
   timeOffset_type GetTimeOffset( ) const;
   fShift_type GetFShift( ) const;
@@ -146,6 +146,7 @@
 %attribute( FrAdcData, nBits_type, nBits, GetNBits )
 %attribute( FrAdcData, bias_type, bias, GetBias )
 %attribute( FrAdcData, slope_type, slope, GetSlope )
+%attribute( FrAdcData, std::string, units, GetUnits )
 %attribute( FrAdcData, sampleRate_type, sampleRate, GetSampleRate, SetSampleRate )
 %attribute( FrAdcData, timeOffset_type, timeOffset, GetTimeOffset, SetTimeOffset )
 %attribute( FrAdcData, fShift_type, fShift, GetFShift )