Multiple Origins, processing input files, documentation

• Added the possibility to have multiple Origin objects in the created file.
  The first added Origin is automatically used as the defining Origin of the file, but an alternative Origin reference
  can be set for each created DLIS object. See examples/create_synth_dlis.py.
• Made file_set_number an optional argument for creating Origin;
  instead, it's generated randomly in accordance with the RP66 standard.
• Added scripts to write data from .xls/.xlsx, .csv, and .las files to a basic DLIS
  (in addition to the preexisting .h5/.hdf5 converter).
  See the dlis_writer/file_format_converter subpackage
  and the related example: examples/create_dlis_from_data.py.
• Added quotes from RP66 to docstrings of add_<object> methods of DLISFile.
• Implemented additional restrictions specified by the standard - e.g. dimensionality of values, number of zones vs
  number of values, number of axis coordinates vs dimension/value, etc.
• Added the possibility for LongName objects to be referenced by Channel, Computation, and Parameter.