Child len context

[wolflu05]

This pr adds two new len attributes to the extended Jinja2 context which can be useful for giving the dimension a padding so that they are all equal length.

• len - count of elements this child will generate
• dim.<x>.len - count of items the x-th dimension has

Example

{{dim.1.zfill(dim.1.len|string|length)}} can be used to pad the first dimension so that they are all of equal length

resolves #35

[github-actions[bot]]

Coverage Report

	Name	Stmts	Miss	Cover	Missing
inventree_bulk_plugin/BulkGenerator/BulkGenerator.py	111	0	100%
inventree_bulk_plugin/BulkGenerator/dimensions.py	66	0	100%
inventree_bulk_plugin/BulkGenerator/generators/gen_alpha.py	36	0	100%
inventree_bulk_plugin/BulkGenerator/generators/gen_numeric.py	14	0	100%
inventree_bulk_plugin/BulkGenerator/generators/generator.py	33	0	100%
inventree_bulk_plugin/BulkGenerator/template.py	10	0	100%
inventree_bulk_plugin/BulkGenerator/utils.py	2	0	100%
inventree_bulk_plugin/BulkGenerator/validations.py	53	0	100%
inventree_bulk_plugin/InvenTreeBulkPlugin.py	122	0	100%
inventree_bulk_plugin/migrations/0001_initial.py	6	0	100%
inventree_bulk_plugin/models.py	21	0	100%
inventree_bulk_plugin/version.py	1	0	100%
TOTAL	475	0	100%