Removed round option

The returned value is now always rounded to the nearest value which creates
better values.
Splitted file BKIntervalState.c into BKInterpolation.h and BKInterpolation.c

Also moved test code into BKInterpolationTest.c
Moved constants into header file

Constant name appears in function description but was not visible in header.
Added description for `maxValue`

Copied description for `maxValue` from other comment.