Fix 64-bit modbus sensor register reads

[tomilehto]

Description:

Previously when reading four 16-bit modbus registers as a sensor
value, slave output was stored first as 64-bit integer, but before returning
that value was converted to double precision floating point. This
caused rounding errors for integer values bigger than 2^53.

After this change floating point conversion is done only if user
has configured scaling or offset to use floating points. In the case
where scale and offset are integers (also the default) only integer
arithmetic is used and 64-bit register values will be read correctly.

This PR adds also unit tests for modbus sensor value conversions
that were missing completely.

Related issue (if applicable): fixes #25671

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#10043

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: modbus
    registers:
     # default good
      - name: nilan_date
        slave: 1
        register: 4722
        register_type: holding
        count: 4
        data_type: uint
     # still good with integer scale and offset
      - name: nilan_date
        slave: 1
        register: 4722
        register_type: holding
        count: 4
        data_type: uint
        scale: 1
        offset: 86400
     # bad when floating point scale or offset
      - name: nilan_date
        slave: 1
        register: 4722
        register_type: holding
        count: 4
        data_type: uint
        scale: 1.0
        offset: 0.0

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.

[tomilehto]

All review comments addressed and checks pass.

[MartinHjelmare]

Please don't squash commits after review has started to make it easier for readers to track changes.

[tomilehto]

Please don't squash commits after review has started to make it easier for readers to track changes.

Ok. I guess I am too used to using gerrit.

[MartinHjelmare]

Looks good!

[MartinHjelmare]

Can be merged when build passes.