DFMeta file handler

[bokov]

Implement the following logic, maybe at the start of DFMeta.init() but maybe in a separate function (pseudocode):

Accept inhandle
  if inhandle is None: 
    try: get the header and meta from those respective args
    except: valueError
  else:
    if inhandle is str and len(inhandle)>0:
      if file exists str: inhandle = filehandle(inhandle);  intype = 'file'
      elif URI resource exists str: inhandle = urihandle(inhandle); intype = 'uri'
      elif valid DB connection str: inhandle = dbopen(inhandle); intype = 'db'
      else: valueError
    # now inhandle should be an actual handle to some resource
    if intype in ('file','uri'):
      if filetype(intype) == 'tsv/csv': parse out headers and meta
      elif filetype(intype) == 'sqlite': pass to datafinisher and then read its output file (update inhandle)
      # future supported formats here
      else: valueError
    # datafinisher cannot yet read connections, only db files
    elif intype == 'db': pass to datafinisher and then read its output file (update inhandle)??
    else: valueError

Now, header, meta, and inhandle are used to init DFMeta. inhandle is optional at init as long as the other two are provided somehow.

[bokov]

Done! Merged into integration and master