account_dunning_letter: Group dunnings by company and party

issue6760

[reviewbot, 2018-05-24 12:57:34]

flake8 OK
URL: https://codereview.tryton.org/48391002

[ced, 2018-05-31 16:15:56]

I find it too bad to loose the company headers.
I think it will be better to make the report single and that the records of
Report are a list of tuple company and dunnings. This way we will have one
report per company with all the dunnings.
This technique could also be used on other company reports and so we keep the
current behavior for mono-company setup.