Store lon_indices in plot driver so more complicated plots are feasible

[Joern Ungermann]

A more complicated plot of mine requires data from the 850 and 500 hPa level. One can implement that rather straightforwardly, if the lon_indices needed for resorting the data were stored during the conversion.

[Reimar Bauer]

Made the lon_indice a member in MPLBasemapHorizontalSectionStyle

Some very hacky plots combining data from multiple levels require partial replication of the data retrieval. To properly do the shifting in longitude, one needs access to the lon_indices.

Fix issue #428

→ <<cset f0f169655972>>