UniversalDopplerMetric: try to compute metric with positive eigenvectors

- new DopplerMetricParams member 'nonposEigValThresh' gives (when >0)
  threshold on number of non-positive eigenvalues that will be tolerated
- integration parameters 'epsrel', 'epsabs', 'Tseg' are now variables passed
  in thru intparams_t; starting values are now set by calling functions
  XLALDopplerPhaseMetric() and XLALComputeAtomsForFmetric()
- XLALDopplerPhaseMetric() makes 32 attempts to compute the metric; after
  each, if 'nonposEigValThresh'>0, the eigenvalues of the (diagonally-
  normalised) metric are checked; if too many are non-positive, error
  tolerances and integration time segment length are slowly decreased
- refs #1250
UniversalDopplerMetric: limit length of integration time segments to >=1800s

- reducing integration time indefinitely can make integrals of short
  observation times take a very long time to (usually) not converge
- refs #1250