Document multiple operations in imeta()

[montesmariana]

The definition and documentation of imeta() suggest that the operations argument requires a named list with "operation", "attribute", "value", "units".
However, it also works with a list of such lists, and the body of the function tries to determine whether it's a list or a list of lists.

I would suggest:

• Change the definition to operations = list(), without names.
• Adapt the documentation so that it's clear that the list is a list of lists, each with at least 'operation', and optionally 'attribute', 'value' and 'units'. (Or should the optionality be different?)
  • The code would internally take care of turning the missing values to NULL.
  • The code would check and force that operations is a list of lists.
  • Eventually it could work with dataframes (one row per AVU, and columns 'operation', 'attribute', 'value', 'units')

If you agree, I can make the changes (at least to the documentation).