- change find() API to make it more consistent
  - whatever the number of variables, always returns a list of dict with the
    variables as key
  - the param 'vars' is now optional.
  - if no variable is passed, search for all variables by default
  - anonymous variables (starting by `__`) are now named var1, var2,... in
    the results
- removed unused `minimalkb/services` sub-package