Rename rap to rapidity

This is the same method name as ScikitHEP's Vector package
Add a note that we adopt the ROOT convention of returning a negative
mass for space-like 4-vectors

README.md

@@ -10,7 +10,7 @@ you can also use `fromPtEtaPhiE(pt, eta, phi, energy) --> LorentzVectorCyl`.
 and these functions for both of them:
 
 ```julia
-px, py, pz, energy, pt, rap, eta, phi, mass
+px, py, pz, energy, pt, rapidity, eta, phi, mass
 ```
 
 as well as these utility functions:

src/cartesian.jl

@@ -1,5 +1,6 @@
 Base.zero(lv::T) where T<:LorentzVector = T(0,0,0,0)
 mass2(lv::LorentzVector) = dot(lv, lv)
+"""mass value - returns a negative number for spacelike 4-vectors"""
 mass(lv::LorentzVector) = mass2(lv) < 0.0 ? sqrt(-mass2(lv)) : sqrt(mass2(lv))
 pt2(lv::LorentzVector) = muladd(lv.x, lv.x, lv.y^2)
 pt(lv::LorentzVector) = sqrt(pt2(lv))
@@ -30,7 +31,7 @@ end
 const η = eta
 
 """Rapidity"""
-function rap(lv::LorentzVector)
+function rapidity(lv::LorentzVector)
     pt_squared = pt2(lv)
     abspz = abs(pz(lv))
     if (energy(lv) == abspz) && (pt_squared == 0.0)

src/cylindrical.jl

@@ -77,7 +77,7 @@ function fast_mass(v1::LorentzVectorCyl, v2::LorentzVectorCyl)
 end
 
 "Rapidity"
-function rap(lv::LorentzVectorCyl)
+function rapidity(lv::LorentzVectorCyl)
     num = sqrt(lv.mass^2 + lv.pt^2 * cosh(lv.eta)^2) + lv.pt * sinh(lv.eta)
     den = sqrt(lv.mass^2 + lv.pt^2)
     return log(num/den)