v0.7.0

Changelog:

• Use Gunicorn to serve requests in production, rather than relying on the builtin Flask server explicitly not intended for production use.
• Fix an issue whereby derivatives were simplified despite an attempt at disabling this. There is now a global constant to enable/disable this, which needs refactoring as an argument to check(...).
• Fix a bug when checking differential equations that contained multiple derivatives of different orders, where the inner lower-order derivatives were replaced too early.
• Correctly parse relations (=, <, <= etc) rather than replace them using a regular expression.
• Refactoring of unit tests to separate out functionality.
• Update to use SymPy version 1.2 and newer versions of other dependencies.