Add check in finpack.core.loader:loader for str in date column

[RackReaver]

I want to catch the error before it is imported using the Account class into the account list. The data brought in from any date column should be an int or float (it is converted to float no matter what) so it will break somewhere in the code if its a str, I want the break point to be in the loader function.

branch suggestion: docs/add-str-check-to-loader
commit suggestion: docs: :memo: #53 - <commit message>