Added a substruct of TsvData called CoilData

CoilData inherits from TsvData and uses the same main method to read in
the data, but it's limited to files with three columns. The advantage is
better semantics and that you can access its variables like

cd->r_locs[i]
cd->currents[i]

rather than
td->data[0][i]
and
td->data[2][i]

However the method I use to construct a CoilData is really ugly but I
don't know a better way :(