Add function scv_data

At the moment it works the same as scv_front, but provides a better way
to access the data in case scv_front is changed to not handle empty
vectors.
Add function scv_clear
Use scv_data in example
Reorder functions

Trying to group functions better.
Use scv_data in second example code
Use scv_data in unit test
Update readme

State that the interface of scv is modeled after C++ std::vector.