ast: Allow FitsChan default options to be set by an env. variable

Any attribute settings specified by environment variable FITSCHAN_OPTIONS
are applied to all created FitsChans, before applying any attributes
specified via the "options" argument of the constructor.
ast: Optimise cache access to avoid valgrind warnings
ast: Fix bug getting ellipse centre
ast: Fix bug in use of CarLin attribute

If the header being read contained a spectral axis, then a value of 1
could be used for CarLin, regardless of the value set in the FitsChan.