v0.5.10

• function merge_npz restored to earlier implementation; use more
  advanced data structures to merge databases where values are not numeric or
  not all databases contain the same set of keys
• function split_del now has additional argument force to overwrite existing
  output files
• function split_del added to module data_import
• fixed a bug that lead function approx_hertz_rad to return radius nan
  rather than inf for a straight line profile. added corresponding test
  case
• function approx_hertz_rad now returns additional parameters
• function abbottfirestone moved to module rough_surfaces
• function import_del and import_dir now have an additional argument
  colheadlines for files where the column headers span multiple rows
• bug fixes