o fixed: 'write_ts_table' with option 'add' would
  not rewrite (i.e. delete) data before 1 Jan 1970

o write_ts_table: new argument 'replace.file'

o write_ts_table: scientific notation is no longer
  suppressed, i.e. numbers may now be written as
  e.g. 1e10

o read_ts_tables: new arguments 'read.fn' and
  'frequency'

o read_ts_tables: 'return.class' may also be
  'ts_table'

o function 'ttime' is now exported

o there are public repositories at
  https://github.com/enricoschumann/tsdb and
  https://gitlab.com/enricoschumann/tsdb