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
```

Since this takes a sequence of 0's and 1's, you can just do:
```
return "".join(str(bit) for bit in sdr)
```