Irregular lattice data blocks do not conform to specifications

[GPMueller]

For irregular lattices, the specs say the position of each data point should come right before the values, effectively increasing the number of columns by 3.
This is currently not respected by the library, both in writing and parsing.

Questions:

• should two data pointers be used or only one, which points to a contiguous block of both positions and values?
• how should the data reading function work: 1:1 into the array or e.g. first read positions, then values?
• do the specs in the Readme need improvement?