Extend schema to allow specification of default values for vars

[dosumis]

It would be useful to have a way to specify default values for vars, with DOSDP_tools allowing TSVs that lack columns for vars with default values (A bit like kwargs in Python) and also allowing for the cells in columns for vars with a default value to be empty.

See INCATools/dosdp-tools#136 for an example use case with a data_var. This could also potentially be useful for patterns where compositional entities are allowed, but not always used. In that case a regular var could specify a default general differentium for the compositional entity (e.g. where a an anatomical entity may be X part_of some Y that default for Y could be 'organism'. Could be very generic

default_value: { some value }

[matentzn]

Mild chase. :)