Added methods to set and add bytes and str

[mjhouse]

Should resolve #31

• Simplified get to allow conversion into any type that implements TryFrom<&DataWord>. This allows us to use the method to get strings back from indexed words.
• Added set_string and set_bytes to update the message body after construction.
• Added add_string and add_bytes to append to the message body .
• Added with_string and with_bytes to parse strings and bytes into words during construction.
• Changed at to return a reference to be more in line with std
• Added validation check for command words to insure that all data words have been given.
• Added tests for all methods

[github-actions]

Coverage

File	Coverage
All files	99%	✅
src/message/packets.rs	100%	✅
src/errors.rs	100%	✅
src/word/words.rs	100%	✅
src/fields.rs	100%	✅
src/message/messages.rs	98%	✅
src/word/enums.rs	100%	✅
src/flags.rs	100%	✅

Minimum allowed coverage is 95%

Generated by 🐒 cobertura-action against b333ed2