Add documentation and testing for natlog interface

    also modified the intents for the f2py generated wrappper
Initial work with constant.f. Test fails.

Array in, modified, an returned. Need intent(inout)
Add doc and testing for constant interface

Also modify intents for f2py wrappers

Note: used intent(in,out) as array is "inout"
Merge pull request #18 from GregHat/constant_func

Add doc, test, and intents for _constant_ and _natlog_ interfaces