Modify the trigonometric properties (partial and total (low) degree and order) to work in the same way as the new power series.

Bsically this is the same set of changes adopted in the new power series code:
* the returned degree/order now is an integer (instead of an integral type),
* we use vector<string> instead of set<string> for the partials, and the new positions
  class for the keys,
* the implementation of the trigonometric_series class now looks saner.

As a bonus, trigonometric_series now compiles on Intel and we could remove the
degree_commons.hpp implementation header.