New slot for syntaxa

[kamapu]

This slot will be linked to header. There the variable should be called "syn_synname", where:

• syn_ is a keyword to recognize syntaxonomies
• synname is the name of the syntaxonomy in the slot.

In slot syntaxa each column will represent one syntaxonomic scheme formatted as taxlist object.

[kamapu]

This is currently modified to the prefix syntax_. Following improvements are still required:

• [x] Slot syntax should be restricted to taxlist objects. Data frames can be placed in slot relations
• [x] A function to pass syntaxonomic information to slot header

[kamapu]

Slot is already created.