Add temperature support to light template

[tetienne]

Description:

Currently light template only support brightness. This PR adds temperature support.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#11678

Example entry for configuration.yaml (if applicable):

input_boolean:
  foo_bool_input:

input_number:
  brightness_input:
    min: 0
    max: 255
  temperature_input:
    min: 0
    max: 500

light:
  - platform: template
    lights:
      foo:
        turn_on:
          service: input_boolean.turn_on
          entity_id: input_boolean.brightness_input
        turn_off:
          service: input_boolean.turn_off
          entity_id: input_boolean.brightness_input
        level_template: "{{states('input_number.brightness_input') | int }}"
        value_template: "{{states('input_number.brightness_input') | int > 0}}"
        set_level:
          service: input_number.set_value
          data_template:
            value: "{{ brightness }}"
            entity_id: input_number.brightness_input
        temperature_template: "{{states('input_number.temperature_input') | int}}"
        set_temperature:
          service: input_number.set_value
          data_template:
            value: "{{ color_temp }}"
            entity_id: input_number.temperature_input

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

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

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[probot-home-assistant]

Hey there @PhracturedBlue, mind taking a look at this pull request as its been labeled with a integration (template) you are listed as a codeowner for? Thanks!

[balloob]

Use a guard clause: if self._temperature_template is None: return. Even better would be to not even call this function if not necessary.

[balloob]

Looking great. Ok to merge after final comment addressed.

[tetienne]

Thx @balloob for your review. I've added the guard clause to other methods too.
Sadly, these methods are called directly by async_update. It will increase its complexity if we remove the guard clause from the child methods.