Drop Dataframes (#193)

Looks that package is no more used in the code. Let's drop it.

REQUIRE

@@ -3,7 +3,6 @@ FEMBase 0.1 0.2-
 ForwardDiff
 LightXML 0.4
 HDF5 0.7
-DataFrames
 Formatting
 Logging
 TimerOutputs

src/postprocess_utils.jl

@@ -2,7 +2,7 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 using JuliaFEM
-using DataFrames
+#using DataFrames
 using HDF5
 using LightXML
 using Formatting

test/test_heat_4.jl

@@ -3,7 +3,6 @@
 
 using JuliaFEM
 using JuliaFEM.Testing
-using DataFrames
 
 @testset "two increments, linear solver" begin
     X = Dict{Int, Vector{Float64}}(