Review CTA effective area classes.

GCTAAeff2D
  The read() method now takes a reference instead of a pointer.

GCTAAeffArf
  Rename read_arf() to read(). read() now takes a reference as
  argument instead of a pointer.
Renamed GFitsTable::hascolumn() method to GFitsTable::contains().

Although GFitsTable does not derived from GContainer, it is basically a container class for FITS columns. Therefore, the check for existence of an element should be done using the contains() method.
Review CTA event classes.

Only the protected interface has been affected by the review. Now all FITS arguments are passed by reference.
CTA module code review.

Declare methods inline.
Review CTA observation class.

The caldb argument is now passed as const reference to the response() method.
Declare some GCTAOnOffObservations methods inline.
Review CTA PSF classes.

Some methods have been declared inline, and FITS file access is now always over reference arguments instead of pointers.
Finish CTA module code review.
Use forward declarations instead of includes and add GCTAResponse::retrieve_ methods to reduce duplicate code.

Using forward declarations instead of includes reduces the file dependencies. We should revise the code globally to use only includes whene really needed.

The GCTAResponse::retrieve methods provide type casting so that the relevant references are extracted from the generic pointers.
Add missing include file.