Externals: some external libraries (ab)use

Hi all,

I am planning to insert some external libraries:
- a module for kinds parametrization and portability and _numbers auxiliary/helper procedures_ (essentially for casting numbers to strings and viceversa); I will start from a reduced version of [IR_Precision](https://github.com/szaghi/IR_Precision), however I would like to have your suggestions on other possibilities;
- for the test suite it is useful to have an _odepack_ in oop; I am planning to start a new small library based on the hints of Jacob [oop-runge-kutta](http://degenerateconic.com/object-oriented-runge-kutta-module/), however feel free to suggest other options;
- a plotting library is useful for the rapid check of results; gnuplot is great, but I would like to play with other solutions... like the one of Jacob https://github.com/jacobwilliams/pyplot-fortran

What do think?