adds `parseFCUnit` function to pyphant.core.Helpers module

Docstring:

```python
def parseFCUnit(s):
    """
    Parses the string/unicode s into a float or a Quantity.

    Returns a float or a Quantity or raises an error.
    See quantities package for possible errors.
    """
```

also adds tests
fixes parts of #68 (input of units broken ...

... in ImageLoaderWorker and probably elsewhere)

* fixes the parameter input for the ImageLoaderWorker
* adds tests
Merge pull request #73 from aheld84/master

fixes parts of #68 (input of units broken ...