Removed duplicate from CMake file
Added utility class for Miller indices
Added operators to UncertainValue

UncertainValue now has arithmetic operators for use with double-values
(exact numbers without uncertainty).

Checks for negative uncertainties in constructor were added (will throw
std::domain_error), so PoldiAutoCorrelationCore could be simplyfied
slightly by abandoning explicit checks for negative sigmas.
Added 0-division check to UncertainValue-operators
Added default constructor for MillerIndices
Added class for POLDI peaks

The existing Peak-class does not really fit, since it's explicitly
written for single crystal calculation and it works with UB-matrices
etc, which is not required for the POLDI-related problems.