Change include quotation to avoid system headers
Miscellaneous fixes

- adjust line break from 80 to 100
- grammar fixes
- missing headers
- doxygen documentation updates
- removed vector from hep::plain that was never used
Move vegas_iteration_result to separate file

- rename member `grid` to `pdf`
Fix build system

- do not conditionally depend on libtool and ...
- remove it completely and use static archives instead. This is
  sufficient for the unit tests
Move vegas callback function into separate files
Remove useless --enable-tests configure switch

- 'make check' still works as expected
Rename `grid` to `pdf`