Updates to be able to use footprints at regular release times and changes to make global grid map with a zoom region

Changes to prep_flexpart for new eSTICC datafile including MDM. Changes FLEXINVERT+ to be able to use inqeuality constraint. Added inequality constraint to repository, needs further checking and development (only test for eSTICC methane sims, no CO2 for example.)

Several changes, mostly some tests, changes to avoid 0-concentration at first step of new background field and correction of grid area calculations.

prep_flexpart: changed eSTICC reading to account for additional files and afternoon averages. flexinvert: Switched of scaling with global error, added option replace a substring (like YYYY) multiple times in same string, added solution for monthly background fields in annual subdirectories.

Added possibility to read and average several footprints belonging to a specific (averaged) observation in FLEXINVERT+. This needs observation files including averaging/measuring time.

Added routines to read NILU (EBAS) data file (test with methane data only)and a netcdf-file with observations for the esticc-project. Started on adding possibility to read FLEXPART background fields in FLEXINVERT.