lfric2lfric: call to init_io

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Until now, in lfric2lfric the I/O context of the source data had to be explicitly initialised, given that the default subroutine **init_io** had dependencies on the **base_mesh** namelist. Now that these base_mesh dependencies have been removed, we can simplify the lfric2lfric code calling the default init_io subroutine.

### Further details of the issue.

_No response_

Start by locating the lfric2lfric entry point and its call to the default init_io subroutine. Check how the source-data I/O context is currently initialized, then simplify that setup using the removal of the base_mesh dependencies; done means lfric2lfric still initializes I/O correctly without the redundant explicit handling.

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