Allows to use different range types for `CoordInterpGrid()`

Allows to use different range types for `CoordInterpGrid()`. For example:
```Julia
x = 2:10              # UnitRange
y = 5.0:0.2:10.0  # FloatRange
z = Float64[sin(i+j) for i in x, j in y]

cg = CoordInterpGrid((x,y),z,BCnil,InterpQuadratic)
cg[2,5.0]
```
Before the same type for `x` and `y` was required.

Also I added `CoordInterpGrid()` to the Readme.