Fix for Undefined index: _regular_price, _sale_price, _tax_status, _tax_class, _visibility, _purchase_note

Undefined index appears for: _regular_price, _sale_price, _tax_status,
_tax_class, _visibility, _purchase_note, when fields are empty, for
example.

Notice: Undefined index: _tax_class in
woocommerce\admin\post-types\writepanels\writepanel-product_data.php on
line 678
Work on charts/reports
flot min
More charts work
Reports + sparklines
Better averages, and fixed stats for the day
class based reports - top sellers and earners, added cache based on query hash
Sales by product
By cat
by category
Hide report for now.
Hint on documentation of function get_product()

The hint actual says that return void, changed to WC_Product_Simple because $woocommerce->product_factory->get_product() returns WC_Product_Simple, this change is useful for IDEs autocomplete hints
Reports
Duplicated German language files for Swiss German locale
Replaced ß with ss for Swiss locale
Replaced Bundesland with Kanton for Swiss locale
Converted .po files to .mo files with msgfmt utility
Update proposal hint on documentation of function get_product()
Update proposal hint on documentation of function get_product()

Update proposal hint on documentation of function `WC_Product_Factory::get_product()` from `void` to an generic `WC_Product`
Update woocommerce-admin-status.php

Added theme info to System Status