holoimage, magdata, phasemap: Made plotting more flexible (now return axes)

phasemapper: new function get_kernel() for convenience, mag_real_fast() for
calculation of the phase with pre-computed lookup-tables
projector: single_tilt_projection() now works as intended
Added scripts for the creation of core-shell-structures
Added script for charge calculation by contour integration
Added script for creating tilt series of core-shell-structure for reconstruction
Various minor modifications, mostly nomenclature and streamlining