Nathan Kelley-Hoskins / gammalib

(I chose `centre` because there were more `centre` than `center`
around.)

The coding and design conventions (CDC) document was also updated.

Some arguments that were not already passed as const references are now declared as const references. Inline methods were added to GCvs.

The configure option --enable-warnings now sets the definition G_WARNINGS in the config.h file.

The function gammalib::warning() has been added to GTools. The function dumps a warning to the console in a standard format if the warning option is enabled.

The function is now used in GFitsHeaderCard to dump any warning about invalid header card value types to the screen.

As arccos() and modulo() are mathematical functions they are better placed in GMath.

All constants are now defined within the gammalib namespace.

The indent parameter allows to signal whether a line is indented. The total length of the parameter name is reduced by the amount of indentation.

The sec_in_day constants gives the number of seconds in a TT day.