Issue #136 Implement summary for cta tapepool ls command

Partially implemented.

The Catalogue::getTapePools() method called by the CTA front-end
now returns an std::list of TapePool structures, where each
structure now has the following additional "summary"
member-variables:

  /**
   * The total number of tapes in the pool.
   */
  uint64_t nbTapes;

  /**
   * The total capacity of all the tapes in the pool in gigabytes
   * (10^9 bytes).
   */
  uint64_t capacityGigabytes;

  /**
   * The total amount of compressed data written to all the tapes
   * in the pool in gigabytes * (10^9 bytes).
   */
  uint64_t dataGigabytes;

These three additional member-variables can now be used by the
CTA front-end to provide the summary information requested by
issue #136 in the appropriate format / presentation.  For example,
the CTA front-end could subtract dataGigabytes from
capacityGigabytes in order to calculate the approximate amount of
free space.  Please note that this calculation could potentially
result in a negative number due to the fact that capacityGigabytes
is an approximate number entered by a tape operator for each tape
when they register those tapes into CTA.  In such a case the CTA
front-end should display 0 and not the negative result.