added loader class and mapping for OEWS data

[ming-afk]

resolves #1 . Scripts included here.

[thomaslin2020]

• Please modify the code so that instead of reading the excel file locally, read it from the url. This also means that we don't need to add too many custom rules in .gitignore.
• What's the problem with repetitions in OCC_TITLE? Would O_GROUP be important for our use cases?

[ming-afk]

• with the current method I manually downloaded the excel data. The data updates once a year, would it be unnecessary to read the url as we are good for one year now?
• The data looks like below. For example the "Chief Executives" position has different O_GROUP but all other data being the same. Case where the O_GROUP and other data are both different is rare. But I'll check the meaning of O_GROUP further to ensure I understand it correctly.

[thomaslin2020]

I think it's still good to have files downloaded through requests as data files aren't easy to share with github