User age/metallicity bounds not respected

[Alex Drlica-Wagner]

The isochrone constructor currently disregards user-specified bounds on age/metallicity and sets them to the range of the isochrone grid (see here). Possible improvements:

1. Continue this behavior but warn the user
2. Respect the user's bounds when they are within the isochrone grid; when outside warn the user and set to the isochrone grid bounds.
3. Respect the user's bounds and throw and exception when they are outside the allowed grid bounds.

For both 2. and 3. above, the default isochrone grid consists of age = [10, 12] and z = [0.0001,0.0002], so the default bounds should be similarly constrained.