skeleton for supporting in-built funcrtions
Support for in built functions: acos(), pow(), sqrt(), fact(). Support for constant PI.
Corrected implementation of lex and parser to read binary operators correctly.
Corrected LALR execution at exec env. Support for precendence with '(' ' )'
re-implemented support for arrays, vectors. fixes #4
Fix #5 (Corrected parsing for if-elseif-else structures)
Adding missing functionalities. Corrections made to execBIn calls. "And now the Mammoth breathes."
Merge pull request #6 from Ritaja/newExecEnv

New exec env