Removal of magic numbers in KeplerSTM_C

[cgyurgyik]

Is your feature request related to a problem? Please describe. On a number of occasions there are magic numbers used for each of the KeplerSTM functions. For example, if (counter == 1000) ... In this case, what is 1000? Or, return -2;. What is -2? By replacing these with names, code readability will improve for future maintainers.

Describe the solution you'd like

• Replace 1000 with a function argument named convergence_limit or convergence_bound.
• Replace the magic number returns with an enumeration:

  enum KEPLER_STM_STATUS {
  FAILURE_TO_CONVERGE_ON_TIME_STEP = -2,
  FAILURE_TO_CONVERGE_ON_CONTINUED_FRACTION = -1,
  SUCCESS = 0,
  };

  This also brings into question, do we actually care where it fails to converge, or is that for debugging purposes only?

[dsavransky]

Fold into #333