Make a QGIS system for underground survey legs

[goatchurchprime]

This is closely related to issue #40 (and potentially could be combined, if not too big).

Currently there is an output from the aven dxf output that has been inputted into QGIS in the directory qgis_resources/centre-lines, but it's not ideal.

@patrickbwarren has a svx/3d to shape-files using plugin that works only in Python2, but it does create cross sections and altitudes. https://github.com/patrickbwarren/qgis-survex-import

The 3D orthographic capabilities in later versions of QGIS could provide a replacement for using Aven, but connected to more features, such as to be found here: https://gis.stackexchange.com/questions/41701/is-there-qgis-plugin-to-allow-3d-visualisation-of-geological-borehole-data-simil