NODES and PROCS variables in export headers

[PierreS-alpha]

Address issue #178

• new function add_export_metadata
• In the configuration of the PROC, new keys:
  • EXPORT_HEADER_NODE_KEYLIST
  • EXPORT_HEADER_PROC_KEYLIST

Implemented only for VECTORS at the moment

example:
In the conf:

EXPORT_HEADER_NODE_KEYLIST|NAME,GNSS_9CHAR,LAT_WGS84,LON_WGS84,ALTITUDE
EXPORT_HEADER_PROC_KEYLIST|ITRF_REF,VELOCITY_REF,VECTORS_VELOCITY_REF

In the ouput header:

# NODE.NAME:  "La Desirade (Guadeloupe)"
# NODE.GNSS_9CHAR:  DSD000GLP
# NODE.LAT_WGS84:  16.308
# NODE.LON_WGS84:  -61.0602
# NODE.ALTITUDE:  272
# PROC.ITRF_REF:  ITRF20(CF)
# PROC.VELOCITY_REF:  0,0,0
# PROC.VECTORS_VELOCITY_REF:  12,15,0