Fold wttrms and Λ into trms

• reformulate the LinearMixedModel type by incorporating the wttrms and Λ members into the trms member.
• create AbstractTerm with subtypes MatrixTerm and FactorReTerm
• add some benchmarks using the BenchmarkTools package
• remove some of the instances of method definitions for functions from Base with signatures of Base classes only
  • BLAS-like in-place linear algebra with scalar multipliers are now called e.g. αβA_mul_Bc!
  • the remaining problematic methods are operations with Diagonal for which I plan to create a PR on the julia repository after consulting with Tony and Andreas