Parameter 'slot' for 'summary()' method

[kamapu]

When applying summary() a parameter called 'slot' may become more comprehensive:

• For summary on taxonomic lists as summary(obj, slot = "species")
• For summary on a relation table as summary(obj, slot = "relations", relation = <a-relation>)

Since a slot syntax is planned, it may work in the same way as for species.

Each slot should have a method.

Relations may summarize frequencies per class and only selected columns

This function may use partial matchings

[kamapu]

Done already