Fix to allow using float-span(1, $i) and tidying up

This pull request contains several fixes. I've filed each fix in a separate commit.

## Fix to allow using float-span(1, $i)

`float-span()` used to reset any numeric `$location` to `1`. It prevented from using the mixin in loops, e. g. `float-span(1, $i)`.

I wrapped the code that resets `$location` into an if-clause that only triggers when `$location` does not contain a number.

## Undo spaghetti

#92 was caused by `find-grid()` returning a single element list. It was solved by adding a check for a single-element list to `float-span()`.

I moved the check from `float-span()` to `find-grid()` where it belongs.

## Removed a meaningless check

The second part of this if-clause was useless:

```scss
@if type-of($grid) == 'number' and length($grid) == 1
```

## The final touch

Added some meaningful comments.