Add feeder cacheing

[josephmckinsey]

Solves #75

tests not passing

[AadilLatif]

@josephmckinsey tests seem to be failing

FileNotFoundError: [Errno 2] Failed to open local file 'outputs_build/voltage_real.feather'. Detail: [errno 2] No such file or directory

[tarekelgindy]

So this now uses the default unit of seconds for the run. This works correctly without specifying the "s" at the end when it's run right?

[josephmckinsey]

So this now uses the default unit of seconds for the run. This works correctly without specifying the "s" at the end when it's run right?

From the opendss manual:

You may specify the stepsize in seconds, minutes, or hours by appending ‘s’, ‘m’, or ‘h’ to the size value. If omitted, ‘s’ is assumed. Example: “set stepsize=15m” is the same as “set stepsize=900.” Do not leave a space between the value and the character