[FIX][8.0] l10n_nl_xaf_auditfile_export: export periodNumber

[astirpe]

On an old multi-company database with a lot of companies we ran into this:

:21714:0:ERROR:SCHEMASV:SCHEMAV_CVC_TOTALDIGITS_VALID: Element '{http://www.auditfiles.nl/XAF/3.2}periodNumber': [facet 'totalDigits'] The value '3167' has more digits than are allowed ('3'). :21714:0:ERROR:SCHEMASV:SCHEMAV_CVC_DATATYPE_VALID_1_2_1: Element '{http://www.auditfiles.nl/XAF/3.2}periodNumber': '3167' is not a valid value of the atomic type '{http://www.auditfiles.nl/XAF/3.2}Nonnegativeinteger3'. :21719:0:ERROR:SCHEMASV:SCHEMAV_CVC_TOTALDIGITS_VALID: Element 

The issue is that the ID of account_period (for periodNumber) has more that three digits. This is not allowed as per the XAF Auditfile schema.

As I already proposed for version 10.0 (see #59 (comment)), I'm proposing to export the period numbers (periodNumber) using 3 digits: 1st digit is the last digit of the fiscal year, the other 2 digits are the month (Eg.: period "August 2017" is represented by number 708).

So this is the backport to 8.0 of the periodNumber representation already made for 10.0.

[astirpe]

@hbrunn for this PR, the milestone should be set to 8.0 (and not to 10.0)

[ThijsvOers]

I have tested the fix Andrea made in a 8.0 environment and it works fine.

[luc-demeyer]

I intended to rebase my PR #119 because it conflicts with this change that got merged.
Before I can do this rebase, this change in this merge must be reverted and fixed because this solution is wrong.
Let me explain:

The period code field is a field that can be entered/modfied by the end user.
We do know that the standard accounting wizard generates period codes via the logic 'mm/yyyy'.
But users can change this and I have seen many installs where the periods are called 'yyyy-mm' (makes actually more sense than the standard way for period based sorting purposes).

I think we should keep the logic proposed by andrea but implemented properly (take the month and year of the period date_stop field in order to compose periodNumber in the XAF file.

[astirpe]

@luc-demeyer is ok for me. Will you fix it in your PR?

[luc-demeyer]

cf. #149
As soon as this one is merged I'll rebase my PR 119.