Fixes Modbus scaling factors and updated specification (#86)

* fixed calculation of Modbus scaling factor and unit conversion factor in all interface types.

* added explicit checks for (not) None.

* fixed Modbus write request.

* fixed Modbus string encoding and decoding.

* updated sgr-specification dependency.

* fixed pyproject.toml

* specification default version.

pyserial and Modbus byte order (#82)

* added pyserial to pymodbus dependency.

* fixed Modbus byte/word order.

* updated minimal dependency versions, maintain compatibility with Python 3.9.

* fixed Modbus timeout values.

* bumped docs version. constructor fix of RestApiServiceCall.

Merge pull request #80 from SmartGridready/jsonata

Merge pull request #79 from SmartGridready/mkr-fixes

Bugfixes

Merge pull request #78 from SmartGridready/mkr-fixes

MQTT and Bugfixes

commhandler release 0.3.1.