+ converted swc_record::kind to an ``enum class``

Fixes issue #8.

All tests are adjusted accordingly and swc_read_cells() is now removed.

Notable difference: parse_record() now throws only an swc_parse_error.
In case of an invalid_argument raised by the cell_record constructor,
this is caught and rethrown as a swc_parse_error with line information
attached.

Moved SWC parser's and cell record's implementation details to a
separate file. Also added braces around single-line control/loop blocks.