Add option for associating columns to metadata properties in the csv plugin

[jesper-friis]

As done in the excel parser in OTEAPI, we want a simple way to:

1. select the columns we want to read
2. associate the columns to properties in the data model

Add an option columns that takes a dict as argument, which maps the column number/name in the CSV file to the property name in the data model.

In some cases, it might be more convenient to provide a sequence or comma-separated string with new column names matching the property names. We could consider to also support that. Columns names left out (empty string) would be ignored.

Needed by Simon in BIG-MAP.