Add FormulaChannels for composing formula outputs (#133)

This PR introduces a channel type called `FormulaChannel`, which derives
from `Broadcast` channels, and a receiver type for it called
`FormulaReceiver`, which derives from the Broadcast `Receiver`.

LogicalMeter now returns `FormulaReceiver`s instead of broadcast
`Receiver`s, and these `FormulaReceiver`s can be composed to create new
formula engines that operate on the outputs of other formula engines.

This would make it easier to build complex formulas that operate on
resampled component data streams. For example:
``` python
net_power_recv = (
    (
        logical_meter.grid_power()
        - logical_meter.battery_power()
        - logical_meter.pv_power()
    )
    .build("net_power")
    .new_receiver()
)
```

Author: sahas-subramanian-frequenz

.gitignore

@@ -39,6 +39,7 @@ pip-delete-this-directory.txt
 
 # Unit test / coverage reports
 htmlcov/
+.htmlcov*/
 .tox/
 .nox/
 .coverage

RELEASE_NOTES.md

@@ -10,7 +10,8 @@
 
 ## New Features
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- Ability to compose formula outputs into higher-order formulas:
+  https://github.com/frequenz-floss/frequenz-sdk-python/pull/133
 
 ## Bug Fixes