NoMethodError: undefined method `arctan' for Math:Module

```
[1] pry(main)> a = Geospatial::Tiles.new(nil)
=> #
[2] pry(main)> a.unmap(3414, 2282, 12)
NoMethodError: undefined method `arctan' for Math:Module
Did you mean? atan
from /home/michael/.rvm/gems/ruby-2.6.5/gems/geospatial-1.14.1/lib/geospatial/tiles.rb:49:in `unmap'
```

Also the `unmap` method should really convert the inputs to floats:
```
[2] pry(main)> a.unmap(3414, 2282, 12)
=> [-180.0, -20.13847031245115]

[3] pry(main)> a.unmap(3414.0, 2282.0, 12)
=> [120.05859375, -20.13847031245115]
```