Corrections for XIRR implementation to achieve more accurate values

Summary:
In the Investment Report, the XIRR() function is used to calculate
the annual return. The previous implementation contains some errors
that have been corrected with this commit. In addition, if XIRR
cannot calculate the annual return to notify the user of this case,
an empty column is displayed in the investment report.

The basis for this fix is an update of the XIRR implementation from
the KOffice project [1], which brings the calculation up to date and
a further improvement from the libreoffice XIRR implementation [2] to
get more solutions using a two-pass approach. The class CashFlowList
has been moved to separate files to have a cleaner separation.

The obsolete and unused NPV function has been removed.

Tests for XIRR have been extended to check for recognized issues.

[1] https://github.com/KDE/koffice/blob/master/kcells/functions/financial.cpp
[2] https://raw.githubusercontent.com/LibreOffice/core/master/scaddins/source/analysis/financial.cxx

BUG:385993
FIXED-IN:4.8.3, 5.0.3

Test Plan: compiled and tested on linux

Reviewers: tbaumgart, wojnilowicz

Differential Revision: https://phabricator.kde.org/D16766