Bugfix with Sign test

Merge pull request #23 from atecon/22-wrong-p-value-for-cg-sign-test

correct pvalue for the center of CGSIGN test dist

Internals and GUI related bugfix

• internally replace deprecated syntax, require gretl 2018a because of that
  – fix GUI bugs related to GW test

Version 2.5

• Bug fixes for ForecastMetrics()
• New features