Draft: merged JAMS routines into FORCES

Closes #38 and makes !17 (closed) obsolete (can be closed)

Features:

• merged features from mo_moment.f90, mo_julian.f90 and mo_utils.f90 from JAMS into Forces
• mo_utils:
  • special_value is now pure, allowing it to be called from pure functions
  • added functions isin and isinloc
• mo_string_utils:
  • toupper is now pure, allowing it to be called from pure functions
• mo_julian:
  • date2dec accepts the optional fracday argument as a fallback to missing hh, mm or ss arguments
  • dec2date accepts the optional fracday output argument
  • dec2date accepts units argument specifying reference times as in NetCDF files (see CF conventions)
• mo_moment:
  • absdev[0], central_moment[0], central_moment_var[0], correlation[0], covariance[0], mixed_central_moment[0], mixed_central_moment_var[0], moment[1], std_dev[1], variance[1], accept the ddof specifying the delta degrees of freedom [default]

Notes:

• in mo_julian other calendars were not implemented as this is too much work and there is no use case