Provide additional SchemaError tests

[stijnvanhoey]

As some combinations of test need specific focus, some preliminar Schema-checks should be done:

• combination of empty: True inside an ìf function without a field-wide empty: True:

lifestage:
    empty: True  # as the empty rule has priority, this is required to enable empty values inside the if-loop
    if:
        sex:
            allowed: [male]
        allowed: [adult]
        empty: True

(the concept of if which further differentiate the conditions for empty values to occur)

• the presence of empty: True inside a delimitedvalues specification

list_of_names:
  empty: true               # The whole field can be empty...
  delimitedvalues:
    delimiter: ' | ' 
    empty: true               # NOT POSSIBLE, should raise SchemaError

• as stated in nesting of if-statements inside a delimitedvalues validation #16, a condition referring to delimited-values field is out of scope. We should warn the user for this.

• see if condition handling when term is not in data set #44, terms inside conditions should be present in the data set as well, otherwise an error should be provided