Tabular module should accept minimalistic file format

[blcham]

There is a need to specify and implement minimalistic versions of schema file definitions, i.e. different use-cases how to use schema e.g.:

• define only important columns
• define columns with ordering
• ...

In case the user does not specify any of the minimalistic definitions we should notify him.

An example that does not work now:

:input-table-schema a csvw:TableSchema ;                                                             
                    csvw:column                                                                      
                        [ csvw:name "bookingno_i" ;                                                
                            kbss-csvw:property  :bookingno_i ;                                       
                            csvw:title "bookingno_i" ] .

In order for tabular module to accept this schema, user needs to specify a csvw:Column type as follows:

:input-table-schema a csvw:TableSchema ;                                                             
                    csvw:column                                                                      
                        [ a csvw:Column;                                                             
                            csvw:name "bookingno_i" ;                                                
                            kbss-csvw:property  :bookingno_i ;                                       
                            csvw:title "bookingno_i" ] . 

[blcham]

We should consider SHACL validation here / inference.