#53 sahko-command improvements and fixes

reworked how nordpool_service works. Cache now contains price data points, not preprocessed price table and graph. Cache is refreshed if it does not contain requested dates data
Fixed command_sahko that it always shows latest data on each message refresh
Added tests for the buttons and showing always latest data when message is updated
Now width of the graph can be set to be anything between 1 to 24 characters. Added interpolation that calculates new data points when graph is requested in a width smaller than 24
Every time graph width is changed new width is saved to chats settings as that chats default width
Added missing tests that confirm that all buttons are shown correctly. Added tests that confirm that the table or graph end result have not changed with the same test data.

M4R774 / bobweb2