String conversion to binary array

NuPIC Network API lacks of a mechanism to read binary arrays from a record and pass them to  PassThroughEncoder. The problem is that FileRecordStream only accepts 'string', 'datetime', 'int', 'float', 'bool' for record data types in CSV files.

This PR brings a simple solution to this. We simply use `sdr` as data-type for the binary array:

```
Square
sdr
T
000000000000000000000000000000000110000000000000011000000000000000000000000000000000000000000000
000000000000000000000000000000000000110000000000000011000000000000000000000000000000000000000000
000000000000000000000000000000000000000110000000000000011000000000000000000000000000000000000000
000000000000000000000000000000000000000000110000000000000011000000000000000000000000000000000000
000000000000000000000000000000000000000000000110000000000000011000000000000000000000000000000000
```

_allowed isn't defined anywhere. You can just change it to this:

```
sdr = [int(c) for c in s if c in ("0", "1")]
```

I also changed `x` to `c` since you are iterating over characters but that is up to you.