Support fixed width files

[hammadk373]

Pandas already support reading from fixed width source files using the read_fwf method. With minor changes to the contract and code, fixed width file support can be added.

[klwhaley]

Included in release 1.1.0 https://github.com/LinuxForHealth/CsvToFHIR/releases/tag/1.1.0