fixed issues with grid spacings

new gridtype: "discrete". This samples the grid points and does not set the probability.

add a fix to distinguish between centred and edge grids when computing number of points over which we should loop and phasevols

also added a warning in the const_dt spacing function to catch the edge case where tmax or max_evolution_time are shorter than the first timestep, which would lead to an empty interpolation table

Decided the chunks / ensemble results per thread will be written as jsons and not as raw strings. Changed tests accordingly

Fixing tests that failed. Added a function to filter dicts based on value instead of key. Added M&S data to grid options rather than custom options

remove deprecated resolution setting

own source files

add option "save_ensemble_chunks" to make sure the ensemble chunks are saved : I am chasing up a grid that completely froze (and gave segfaults) ...

add option to convert dict keys to floats when loading ensemble data : this is required for matplotlib/seaborn otherwise the plots are completely messed up