FlatLatticeTiling: comments/whitespace/minor code cleanups
FlatLatticeTiling: relocate/rename internal functions, with minor code cleanups
FlatLatticeTiling: privatise XLALMetricEllipseBoundingBox() and XLALMetricLatticeIncrements()
FlatLatticeTiling: replace lattice generator functions with enumeration

- enum FlatLatticeType lists the possible lattices
- XLALSetFlatLatticeGenerator() and XLALSetFlatLatticeMetric() merged
  into one function XLALSetFlatLatticeTypeAndMetric()
- lattice generator code moved to FLT_MetricLatticeIncrements()
FlatLatticeTiling: change method of total template counting

- store total count in tiling struct within XLALNextFlatLatticePoint(),
  so any iteration over all templates will store total count for later use
- rename XLALCountTotalFlatLatticePoints() to XLALCountFlatLatticePoints()
FlatLatticeTiling: pass in tiling structure as 'const' where possible