Introducing armdillo in PadeApproximant.

- throw away petsc
- create method for evaluating coeficients of pade approximant
- no working code but compiles
Improve factorial function.
Compute Pade approximant using armadillo instead of petsc.

- convert all methods to use armadillo
- check the test 17 - results are the same
- parallel run is now without petsc error
PadeApproximant cleanup.

- improve comments methods and members
- remove old code
- rename members
PadeApproximant matrix assemble transpose (as in LinearReaction).
Compute factorials in PadeApproximant only once, remove method factorial().
Just create branch PE_pade_approximant
Merge pull request #198 from flow123d/PE_pade_approximant

Further work separates Pade Approximant as numerical method and assembly of the system matrix of ODE resulting either from decays or from reactions. Resolve issue #136