[FIX] Error when manually adding analytic account in the generated tax lines on an invoice

fixes #374
fixes https://bugs.launchpad.net/ocb-addons/+bug/1084822

The fix considers invoice tax lines with different analytic account
are equivalent for the purpose of checking if the list of tax line
is complete.

Caveat, this changes the structure of keys in the dictionary
returned by account.invoice.tax's compute method, I suppose this
is ok for the master branch.
[FIX] Preserve analytic account on tax lines which are on same general account as invoice line

After careful analysis, I'm now convinced it is a good thing to preserve
the analytic account on taxes line which have the same general account
as the invoice line.

This is the best default case and will save time for users,
while leaving the flexibility to adapt the analytic account on
taxes manually.