Add Cost and Tariff Sensors, plus other bug fixes

[ColinRobbins]

Description

• Added Current Cost sensor (Integrating PR Cost sensors #31)
• Added sensor for Tariffs (Standing charge and unit rate) (Feature request: Pull tariff data (price per kw etc) and provide as sensor to calculate cost on Energy dash #24)
• Updated Readme

Motivation and Context

Enhancements as above, also fixes...

• Fix re-authentication failure after 7 days Thanks for the good effort but this is not reliable #43 and No data #14
• Fix bug in BST time calculation BST hardwired and possibly Data @ 11pm-12am is lost every night #45
• Fix codes issues (pylint, codeql)

How Has This Been Tested?

Running in my dev and live environment.
Several other users have been using my fork as an interim solution, from which this pull is derived.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[HandyHat]

This looks great. Thanks @ColinRobbins!