Improve readability of `piecewise_constant_pdf`

- include the left boundary of bin zero which is always zero
- right boundary of last bin is always one
- rename the variables inside `icdf` to more meaningful ones
Improve readability of parts of the VEGAS code
Add test for `vegas_adapt_grid`
Adjust code to 100 char limit
Fix little but serious bug
Add a proper test for `vegas_adjust_grid`