SWC file parser.

A very basic parser of SWC files. The role of the parser is to extract
cell records from an SWC file line-by-line, which will then be used to
construct the neuron cell models.

The parser currently does only a simple linewise processing of the cell
records. This practically means that the validity of the input is only
checked at the cell record level.

More advanced input processing, such as detection of duplicate cell ids,
holes in cell id numbering or multiple cell trees in a file are not
currently treated.