Feature Request: Reject MPP payments from send_to_route

**Describe desired feature**

Currently `send_to_route` will accept a route with multiple paths and just send on the first path. To enforce correct usage of this API, we should:
* Error if more than one path is provided
* Note this in docs

**Use case for feature**

Improvement of API.

**Would you like to contribute code for this feature?**

If you're interested and able to contribute code for the feature - let us know!

Review!