Add round to half template method

[tofuSCHNITZEL]

Description:

extended the round function to accept "half" as a method which will round a number to the nearest .5 value.

2.1 | round(1,"half") => 2.0
2.2 | round(1,"half") => 2.0
2.3 | round(1,"half") => 2.5
2.7 | round(1,"half") => 2.5
2.8 | round(1,"half") => 3.0

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#11257

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the [development checklist][dev-checklist]

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

[balloob]

Can you add a test

[tofuSCHNITZEL]

Can you add a test

sure I've added one test. should I add more cases for different values?

[balloob]

Thanks !