Cell record rename

Fixes issue #8.

I think that it is good enough for now to merge this

+ some const specifiers.

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.

Begin iterator resets the input stream.

Adde `-g' flag in compilation to allow debugging.

Use of cell record ranges for hiding the internals of file reading. This is
a proof-of-concept for a first peer review and discussion. Check usage
paradigm in corresponding test. Now we could easily support stateful parsing
reporting context of problematic lines.