Linking to Anaconda environment in Ubuntu

Hi.

Is there a guide on how to direct the Arrayfire installation to an Anaconda environment? I saw a couple of issues here, but they were based on Windows and Mac OS. I'm working with Ubuntu 19.10.

If I try to import Arrayfire in Python I get the following:

```
>>> import arrayfire
Traceback (most recent call last):
File "", line 1, in
File "/home/mario/anaconda3/lib/python3.7/site-packages/arrayfire/__init__.py", line 56, in
from .library import *
File "/home/mario/anaconda3/lib/python3.7/site-packages/arrayfire/library.py", line 643, in
backend = _clibrary()
File "/home/mario/anaconda3/lib/python3.7/site-packages/arrayfire/library.py", line 619, in __init__
raise RuntimeError("Could not load any ArrayFire libraries.\n" + more_info_str)
RuntimeError: Could not load any ArrayFire libraries.
Please look at https://github.com/arrayfire/arrayfire-python/wiki for more information.

```
You can see that it's trying to look for Arrayfire in the `anaconda3` folder, and it is obviously failing because my Arrayfire installation is in `/opt/arrayfire/`.

Any pointers to how I can fix this?

Start with the Python import path in arrayfire/__init__.py and the library-loading code in arrayfire/library.py, then read the linked ArrayFire Python wiki. Reproduce the import failure in the Ubuntu 19.10 Anaconda environment and document the supported setup; done means the guide lets users load ArrayFire from /opt/arrayfire without the RuntimeError.

索引モデルが issue の本文から書いたものです。