Make BoundingBox inputs integer

[larrybradley]

This is a followup to #97 to ensure the inputs to BoundingBox are integer.

Note that int() is called on the inputs, which means floats will be silently truncated. Do we also want to raise an exception for floats where e.g., int(ixmin) != ixmin?

[keflavich]

For consistency with numpy's new behavior, yes, we should raise an exception there. I preferred the old approach of just warning, which I used all the time, but it will probably benefit many people... including me... to be forced to select how floats are truncated.

[cdeil]

+1 to be strict on inputs and raise errors. Numpy now raises warnings where floats are used as indices (because I guess they can't throw errors to keep backward compat).

[keflavich]

@cdeil I'm using numpy 1.12-dev, and I get exceptions if I try to use floats:

In [6]: np.__version__
Out[6]: '1.12.0.dev0+47c18a0'

In [7]: x = np.arange(5)

In [8]: x[2.0:3.0]
Traceback (most recent call last):
  File "<ipython-input-8-82b00ecaa918>", line 1, in <module>
    x[2.0:3.0]
TypeError: slice indices must be integers or None or have an __index__ method

[larrybradley]

OK, done. Input floats (even e.g. 3.0) will now raise a TypeError.

[cdeil]

I'm still on Numpy 1.11 ... there it's a VisibleDeprecationWarning:

In [1]: import numpy as np

In [2]: np.__version__
Out[2]: '1.11.3'

In [3]: x = np.arange(5)

In [4]: x[2.0:3.0]
/opt/local/bin/ipython:1: VisibleDeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  #!/opt/local/Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5
Out[4]: array([2])