Tree.R : R-CMD check

[Klangina]

• generate_fa2tre: no visible binding for global variable ‘tre_path’
• generate_fa2tre: no visible binding for global variable ‘prot’
• generate_fa2tre: no visible global function definition for ‘midpoint’

Note, in addition to undefined global variables, this function will need to declare an import for midpoint
After the changes in PR-#108, the following errors are of significance.

• createFA2Tree: no visible global function definition for ‘midpoint’
• createFA2Tree: no visible binding for global variable ‘fit_optimized2

Originally posted by @the-mayer in #4 (comment)

[Klangina]

@the-mayer : Kindly review the above PR.