refactor(dfns): better dim specification

[wpbonelli]

Replace separate field and expr attributes on dimension specifications with a single value attribute, which may be

• an integer field name, e.g. nlay, or
• len() of a size-unspecified 1D array field, e.g. len(array_field), or
• a more complex Python-syntax expression combining other dimensions

Previously only sum() and len() were supported in expressions, add support for Python builtins as well as math module functions