change single ptr struct to double ptr struct for ctypes for subcatch

[jennwuu]

Closes #211

-Update subcatch_statistics from single ptr to double ptr
-Add an unit test
-TODO: should add a metric unit test in here..

units for the following variables:

• precipitation [volume in depth] [in/mm]
• runon [volume] [ft3/m3]
• evaporation [volume] [ft3/m3]
• infiltration [volume] [ft3/m3]
• runoff [volume] [ft3/m3]
• maxFlow [flow] [cfs/m3/s]

int DLLEXPORT swmm_getSubcatchStats(int index, SM_SubcatchStats **subcatchStats)
///
/// Output:  Subcatchment Stats Structure (SM_SubcatchStats)
/// Return:  API Error
/// Purpose: Gets Subcatchment Stats and Converts Units
{

[bemcdonnell]

@jennwuu, go ahead and merge code and release v0.5.2 (also close the milestone and issues associated). Sloth power

[katmratliff]

Thanks @jennwuu!