mdim convert: support '--isubset' for index (not coordinate) range

[mdsumner]

Feature description

Support a new mdim conversion option --isubset to take either a pair DIMENSION(start, end) or single DIMENSION(pos) 0-based dimension index. This would apply the implied subset across all variables sharing DIMENSION and maintain the naming scheme of the source.
This would be incompatible with the existing --subset and 'view' component of --array inputs

Additional context

Currently we cannot subset variables via dimension index in the cli. We either --subset DIMENSION(min,max) or --subset DIMENSION(val) to specify a range or single slice from coordinate values, and this is applied across all variables sharing that dimension. With --array we can use array indexing but necessarily only applied to one array, and is potentially incompatible with other array definitions so we necessarily get new synthetic dimension names (i.e. #8305).