Check that weight variables exist

[phackstock]

Closes #132.

Features added

The validation of the variable code list now includes a check that all variables mentioned as a weight also have their own entry in the list. I've written the validator so that it collects all missing weight variables and does not throw an error for the first one. This way multiple errors can be fixed in one go.
I added a point in the documentation to highlight that the weight is required.
I also took the liberty to slightly refactor the codelist validators. The main change there is the switch from using root_validator() to validator("mapping").
The reason for this change is that in the validators only modify and validate the mapping attribute. The name attribute is mainly used for better error reporting in this case. The way how to access multiple attributes in a non-root validator is adding v and values to the list of input variables (details can be found under the code example of "Validators" https://pydantic-docs.helpmanual.io/usage/validators/). As an example:

@validator("mapping")
def some_validator(cls, v, values):
    # v is the field to be validated "mapping" in our case
    # values is a dict, that contains all other previously validated fields

If you don't like the switch from the root_validator though I can also revert it.
Looking forward to your comments.

[danielhuppmann]

A nice improvement, thanks! Just a few suggestions for shorter, more readable documentation and error messages.