hardisp static data might be a thread safety issue

[ourairquality]

Noticed in passing that src/cpp/3rdparty/iers2010/hardisp/tdfrph.cpp is using static data for a cache that seen alone might be a thread safety issue, and I see some claims that ginan is intended to be a multi-threaded app. This may be fine, and I know little of the threading design of ginan, the callers might be ensuring this is safe by design or with locking, so please close if not an issue.

  static GTime last_epoch;
  static double d[6];
  static double dd[6];