WIP : Fix 1633 unexpected key error with primary key in schema

[amelie-rondot]

• fixes KeyError when a "primaryKey" is missing #1633

---

This PR fix unexpected KeyError raised when a primary key used in shema is missing in the tabular data to validate.
For example:

data = [["B"], ["b"]]
schema = {
        "$schema": "https://frictionlessdata.io/schemas/table-schema.json",
        "fields": [
            {"name": "A"},
            {"name": "B"}
        ],
        "primaryKey": ["A"]
    }

This PR adds some test cases:

• two test cases to ensure that when a label related to the schema primary key is missing, the validation report is invalid with single missing-label error:
  • a test case with the schema containing the primary key field not specified as 'required'
  • a test case with the schema containing the primary key field specified as 'required'
• a test case to deal with insensitivity case in the data label corresponding to the primary key schema field: the validation report is valid

NB: to deal with insensitivity case, this PR should be rebased the branch related to this PR

Through this PR, I also suggest a refactoring to introduce intermediary TableResource methods to create cells related to primary key schema.

[amelie-rondot]

This PR is replaced by PR#1641