Feature/allow null values in ride points

If a ride file has a series point that doesn't not contain all values GC will import the unset values as 0 or 0.0.  This change will make GC assume that those unset values are the same as the last sample.  This allows programs that record data to generate much smaller files by only including the measured points. 

Ex:
{"SECS":1, "HR":97, "WATTS":152},
{"SECS":2, "HR":90} 

Before:
GC will import the second point with a WATTS of 0.0.

After:
GC will assume that since the second point doesn't set WATTS that WATTS hasn't changed and import 152.