Function:     factor

Purpose:      Factor returns the conversion factor between two different
              units (i.e. M en KM)

Category:     PHYSICAL COORDINATES

File:         factor.c

Author:       K.G. Begeman

Use:          INTEGER FACTOR( CUNIT1 ,     Input    CHARACTER*(*)
                              CUNIT2 ,     Input    CHARACTER*(*)
                              CFACT  )     Output   DOUBLE PRECISION

              CUNIT1     Type of unit to convert from.
                         The units can be abbreviated, i.e enough
                         characters must be given to find a unique
                         match in a list of known units.
              CUNIT2     Type of unit to convert to.
              CFACT      Factor to convert from CUNIT1 to CUNIT2.
              FACTOR     Returns:
                          0: successful conversion
                         51: unknown units to convert from
                         52: unknown units to convert to
                         53: both units are unknown
                         54: incompatible units
                         55: ambiguous units to convert from
                         56: ambiguous units to convert to


Notes:        The following type of units are implemented:
              DEGREE      ARCSEC      ARCMIN      RADIAN
              CIRCLE      DMSSEC      DMSMIN      DMSDEG
              HMSSEC      HMSMIN      HMSHOUR     
              METER       ANGSTROM    NM          MICRON      
              MM          CM          INCH        FOOT        
              YARD        M           KM          MILE        
              PC          KPC         MPC
              AU          LYR
              TICK        SECOND      MINUTE      HOUR
              DAY         YEAR        YR      
              HZ          KHZ         MHZ         GHZ         
              M/S         MM/S        CM/S        KM/S        
              K           MK
              W/M2/HZ     JY          MJY         
              TAU
              JOULE       J           EV          ERG         RY                       

Updates:      Dec 23, 1989: KGB, document created.
              May  9, 1995: VOG, minimal match.
                                 DMSSEC/MIN/DEG,
                                 HMSSEC/MIN/HOUR
              Mar 25, 2009: JPT, improved minimal match
              Mar 29, 2009: VOG, units for energy added
                                 (WCSLIB) and updated value for pc.
                                 Also added au and lyr