creates a new SchemaVariable
Parameters: name - the Name of the SchemaVariable Parameters: sort - the Sort of the SchemaVariable and the matched type Parameters: rigidness - true iff this SV may only match rigidterms/formulas Parameters: listSV - a boolean which is true iff the schemavariable is allowedto match a list of terms Parameters: strictSV - boolean indicating if the schemavariable is declared as strictforcing exact type match