Added tolerances to Coord.is_contiguous()

The ```iris.coords.Coord.is_contiguous()``` method tests for equality between adjacent upper and lower bounds to determine whether bounds are contiguous. This runs into issues with floating point numbers. For example:
```python
import numpy as np
import iris

delta = np.float64(0.00001)
lower = -1.0 + delta
upper = 3.0 - delta
points, step = np.linspace(lower, upper, 2,
                           endpoint=False, retstep=True)
points += step * 0.5 
coord = iris.coords.DimCoord(points)
coord.guess_bounds()
coord.is_contiguous()
```
returns False.

This PR modifies ```is_contiguous()``` to use ```np.allclose()``` adding the tolerance keyword args to ```is_contiguous()```.