Warn in Combine plugin for new variable name not matching CF conventions

[mo-robert-purvis]

The cube_combiner.py plugin Combine allows for a new name to be given to the variable in the combined cube. It will allow for one that doesn't conform to the CF conventions on variable names (https://cfconventions.org/Data/cf-standard-names/docs/guidelines.html) - allowing a name starting with a digit. An invalid variable name will subsequently be silently modified by iris with var_ added as a prefix.

This change to IMPROVER will add a warning to inform that the variable name is contrary to the CF naming conventions, and that Iris will add a prefix to the name. The new_name value is not modified - simply warned about.

The output of this message is as a user warning, e.g.:
UserWarning: improver.api.Combine during execution: The new variable name (10m_ratio_wind_speed_of_gust) does not start with a letter, and so is contrary to the CF convention for variable names. Iris will add 'var_' as a prefix to it.

Testing:

• Ran unit tests and they passed OK
• Tested with an application that uses the Combine plugin and sets a new_name argument starting with a digit