Custom save load

[WojciechMula]

Solves #102

[WojciechMula]

Python 3.6.4

+-----------+--------------------+----------+-------------------+
| function  | peak mem usage [B] | time [s] | file size [B]     |
+-----------+--------------------+----------+-------------------+
| pickle    |      1 757 601 792 | 3.78     |                   |
+-----------+--------------------+----------+       486 464 701 |
| unpickle  |      1 757 601 792 | 1.71     |                   |
+-----------+--------------------+----------+-------------------+
| save      |        584 447 264 | 2.12     |                   |
+-----------+--------------------+----------+       584 447 264 |
| load      |        598 052 864 | 5.87     |                   |
+-----------+--------------------+----------+-------------------+

Features:

• smaller memory usage (3 times smaller in experiments)
• faster save
• slower load
• bigger output file