Removed exception thrown by interpolation
This has the benefit of removing a large proportion of exceptions thrown by the core (see #257 (closed)).
- Invalid interpolation is now handled by state rather than exceptions
- Instrumentstate/InterpolatedState now have a flag _valid denoting whether the interpolation failed
- ReciprocalVector and Intensity are common operations using interpolation, and therefore also have a _valid flag
- All try/catch and thrown exceptions arising from InterpolatedState have been replaced with state checks
Resolves #142 (closed) Replaces !350 (closed)