Add Cumulative Sensor

[ColinRobbins]

Description

This pull adds a cumulative sensor, showing usage since the start of the year.

Motivation and Context

• This is in part a feature request: https://github.com/HandyHat/ha-hildebrandglow-dcc/issues/25
• It also addresses the issue discussed in https://github.com/HandyHat/ha-hildebrandglow-dcc/issues/45, where the "today" sensor misses 30 minutes at the end of the day.

With this fix, the missing 30 minutes will be recorded in the totals for week/month/year seen by the Energy integration, although upto 30 minutes of data may still be recorded against the wrong day. I cannot see a solution to this without using a Hildebrand hardware device.

How Has This Been Tested?

Running in local development environment.

Types of changes

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

[HandyHat]

Looks great. Thanks @ColinRobbins!