Create ForecastValueYearMonth with factory function to avoid warning message

[dfulu]

The way the ForecastValueYearMonth classes are created for each year and month currently causes a warning message when this library is imported like:

warnings.warn(message, UserWarning)
.../nowcasting_datamodel/models/forecast.py:212: SAWarning: This declarative base already contains a class 
with the same class name and module name as nowcasting_datamodel.models.forecast.ForecastValueYearMonth, 
and will be replaced in the string-lookup table.

This pull request creates the classes dynamically and should remove the warning message since the class names are distinct for each year and month

Closes #148

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[peterdudfield]

Ive run migrations locally, and as expected

1. no new migrations casued by this code
2. new forecast_value_YYYY_MM tables can be created as usual