Major changes from v0.1.x to v0.2.0

1. Added memoization to avoid repeated calling of zeta fn for same
   arguments (could add this for some other functions, inc. the polylog itself)

2. Added m-th root formula to avoid deep recursion and improve
   accuracy in the region for large real s and very large z. There are
   two implementations:
     
      - direct
      - reordered sum

   Also checked asymptotic formula again, with predictable poor
   accuracy.

3. Changed output of internal functions to provide better information
   on recursion (so now they return the series being used as a tree,
   and the max_recursion as a direct variable).

4. Change prereqs to make it more flexible, particularly WRT to
   SpecialFunctions.

5. The utilities included code for parsing numbers to allow reading in
   Mathematica output. However: 

   + Rational parsing was added for Julia's rational somewhere around
      1.9 or 1.10, so drop this, and make 1.10 the requirement.
   + Complex number outputs from Mathematica seem to have changed to a
      more standard form, so the code was updated.

  Note that the parsing code isn't a core part of the package, and is
  used only for testing, so maybe it should be removed into the test
  directories? 

6. Many of the tests have been changed to reflect the above, and new
   test data has been added as well as around 100 new
   identities for consistency and special value checking.

7. Added some new functions 

   + Rogers L-function
   + Spence's function
   + Dilogarithm
   + Trilogarithm
   + Tetralogarithm

8. Increased lengths of some sequences

   + First 79 Stieltjes constants (instead of 10)
   + First 60 Bernoulli numbers (instead of 35)
   
