Small refactoring and regular expression simplification in the
timesheet parser. Add simple parser test.

Extract details from email message received after filling online
Barkauf form saved with attachments from Notes mail client.

Instead than generating entries for all documents and then drop the
ones already present in the ledger, immediately skip folders for which
an entry with a corresponding "documentation" metadata entry already
exists. This makes a substantial differnece in run time now that there
is a large number of folders.

Switch from os.listdir() to os.scandir() to speed up directory walking.

The import is quite slow and the module is not always used.

Specialized exceptions make understanding why opening a new SAP
session fails much easier, in particular when it happens because of
inactivity auto logout.

Funds can be transfered between accounts to cover for overbooking or
price fluctuations between quote and invoice. These transfers are
however not recorded in SAP in an accessible way. It is easier to
discard them when comparing account balances. Use the T transaction
flag for this purpose.

This allows to skip closed accounts when looking for new entries.

Export transactions in the desiganted accounts in table format with
the possibility to save the table in CVS or XLSL files.

This allows to assign a date to orders that do not go through a
requisition, for example some framework agreement orders.

Compare SAP and local ledger total balances only.

Populate more fields in the SAP application and do not rely on default
values. The handling of requisitions and orders across the year end
boundary is still fuzzy.

Collects all the order acknowledgements in a temporary directory.

Matching the accounts with a regular expression allows to extract all
the records with only one BQL query thus not requiring multiple
iterations through the transactions in the ledger.

The regular expression could be further optimized, see Emacs
regexp-opt function, but it is probably not worth the effort.

Cumulative balances are more efficiently obtained adding to account
inventories while iterating one time through the ledger transactions.
The only subtlety is that, because inventory objects are mutable,
intermediate inventories need to be copied when are emitted or will be
overwritten when subsequent transactions are added.

Using functools.total_ordering does not work when inheriting from
datetime.timedelta because the class appears to already implement all
the comparison methods.

Avoid massive code duplication with clever(?) dynamic function
generation. The same technique could be used to generate methods
implementing binary functions.

Also add a __neg__() implementation.

Orders are considered placed on the current year budget if the order
acknowledgement is received before the end of the year. This command
allows to check that order acknowledgements are present in open orders
documentation folder. Orders that are already marked as paid are not
considered.