Fixed_Time hardening model: should error for (1) times out of bounds, (2) incorrect interpolation

[lzkelley]

When requesting hardening timescales of 1e-5 and 1e5 Gyr (for example), the resulting interpolated values completely fail to reproduce these timescales and the resulting GWB spectrum is identical.

(1) The code should raise an error for values outside of bounds
(2) The code should raise an error if the resulting interpolated values deviate from the desired hardening timescales by greater than some factor. (Note: currently a warning is raised that the 'normal' interpolant failed, and the 'backup' is being used). Accuracy should be checked for both interpolants.