Make default string size configurable in the model, use in cpp and dictionary codegen

[jwest115]

• Updated dictionary spec to require FW_FIXED_LENGTH_STRING_SIZE (PR: Update dictionary spec to require FW_FIXED_LENGTH_STRING_SIZE fprime#4667)
• Updated cpp and dictionary codegen to use FW_FIXED_LENGTH_STRING_SIZE for string size
  • Note: Since this constant is not required by cpp codegen, I kept 80 as the default if the constant is not in the model.
• Removed -s option from fpp-to-cpp and fpp-to-dict
• Updated user guide

Closes #903.

[bocchino]

This is great work! I'm closing this PR in favor of #913, which is on a branch in the repo. This work is exposing bugs in FppTest, so we need an F Prime tandem development.