sp3 context

[gwbres]

This branch proposes the possibility to load a pool of SP3 files.
SP3 files will permit solving SPP and PPP, and possible other operations.

• Cli: introduce SP3 files pool #152
• Support IGS SP3 files #82
• Support as many SP3 files as desired
• The SP3 lib supports merge(file1, file2): we use this feature to create the total SP3 context
• The SP3 lib supports seamless gzip compressed SP3 file decompression.
  Using this capacity make it very easy to import such data, directly downloaded from data providers like CDDIS
• Currently we only display the precise orbits and the clock predictions (file visualization basically).
  We do not perform calculations that SP3 will enable yet, this will come later.
• Other minor modifications
  • : improved the workspace naming process.
    We want the workspace to be named $WORKSPACE/$FILENAME/, the current implementation creates $WORKSPACE/$FILENAME.CRX when a gz compressed file is given (--fp $FILENAME.CRX.gz), due to the double .
  • Add one documentation page, to sumarize what minimal context should be provided for all possible operations