Add new tests showing cases which were failing before (test34,35,36)

Add also a corresponding test

Added compilation flag  -Wshadow for cling and gcc. Fixes ROOT-7378 Insufficient warning level with CMake

Fix handling of composite polynomial functions. They are not linear and the number should be zero. This fixes also ROOT-7755

Handle also better cases when function name was contained in other name (e.g. sq and sqrt)

Change TMath::Sign for inter types to be a templated T1 Math::Sign(T1, T2). This fixes problem of Sign(1,double(x)) observed in TFormula.
ROOT-7375.
Add also an overload of TMath::Sign for LongDouble

This required some changes in the handling of function shortcuts and the loop in ProcessFormula needed to be split in two.
Add also function "sign" and tests for "sq" and "sign" functions.

-  Fix the syntax of the TFormula constructor to be same as previous version
- Fix a bug in handling polynomials
- Fix another bug in operator ^ when replacing same expressions (e.g. x^10+x^1)

Add as a persistent members the parameter names and their index (order) of TFormula. This makes the order preserved when reading a TFormula from a file.

Add also Chebyshev polynomial as pre-defined functions in TFormula
Add new tutorial for Chebyshev polynomials

Fix Printing of parameters in correct orders.
Add methods to retrieve variable number and name given a number

Add tests for TFormula::Eval

Add option to GetExpFormula. Supported options are "P" to replace parameter names with their values and "Cling" to retrieve the expressions passed to Cling.

Add corresponding tests in TFormulaTests (TFormulaParsingTests.h)