Remove duplicate functionality

[mlt]

There are many chunks of code that essentially do something readily available otherwise. E.g., AddFID can be implemented either with bare OGR (OGRLayer::CreateField), QGIS (QgsVectorLayer::addAttribute), or sextante (AddTableField).

It would be nice to leave the bare bones essential for PIHM such that it can be reused elsewhere like via sextante. Thus, perhaps, OGR would be the best solution for now in aforementioned example as those libraries are used anyway until entire PIHM pre-processor can be modeled via sextante or alike.