apasto/ggm_bg_maps_binder: v1.1

Drastic strategy change: previously, grids where needlessly synthesised from SH expansions, then we performed grid-node-wise operations (i.e. removing the topographic effect from the gravity disturbance). This was both inefficient and required some workarounds.

Main changes:

• reduction for the topographic effect is performed in the SH coefficients domain (using the operator overloading provided by pyshtools classes)
• the only grid synthesis performed is the one from topography reduced field to the relevant grids
• we avoid truncation (the gentle-cut module is left aside), following the Earth2014 Authors recommendation to not do so
• general refactor, leaner environment.yml
• updated documentation (in notebook cells)