Changes the API for recording variables.

Uses a method called 'record' that provides a key value,
instead of providing keys to extract from context.

The variables are stored internally to the algorithm in a dictionary,
and not just stared as a property of the algorithm.

Main intent behind this change is to make the API more user friendly,
since the previous recorded_variables relies on the value to be set
in the algorithms context/self, the hope is that only having to use
the `record` method means less moving pieces and a more understandable
API.

i.e., instead of:

```
def initialize(self):
    recorded_variables('foo', bar')

def handle_data(self, data):
    self.foo = 1
    self.bar = 2
```

The API is now:

```
def initialize(self):
    pass

def handle_data(self, data):
    self.record(foo=1, bar=2)
```