Use of super()

I don't _believe_ the `super` calls are necessary for the subclasses (e.g., [`SampleRepo`](https://github.com/biocore/microsetta-private-api/blob/52798b9823332114aa98b20a939cabdbbbdc1527/repo/sample_repo.py#L6-L8)). If the subclass doesn't override the constructor, then the base class constructor is implicitly called:

```python
$ ipython
Python 3.6.7 |Anaconda, Inc.| (default, Oct 23 2018, 14:01:38)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.0.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: class base:
...: def __init__(self):
...: print("blah")

In [2]: class sub(base):
...: pass

In [3]: base()
blah
Out[3]: <__main__.base at 0x1070bbfd0>
```