Generation of global/core-excised Chandra spectra

[DavidT3]

At the end of this task we should have the ability to make Chandra spectra in global apertures (i.e. from radius of zero to some user-defined outer radius), and to make them in 'core-excised' apertures (from some inner radius to some outer radius, both user-defined).

We'll use specextract, which will be lovely because it'll basically do everything we need.

Some general notes/thoughts:

• We have to remember that, even though we care the most about extended sources, XGA is a completely general use tool - as such, for things like ARF weighting during spectrum generation we have to check the type of source being passed in and change the settings accordingly (you can see we do that for XMM in the spectral generation stage).
• The RMF weighting option is pointless and extremely computationally expensive (thanks @astrophysics-megan !) - we can also probably cut down on the energy resolution of the RMF for more speed.
• The input regions have to be in Chandra detector coordinates (easy to achieve, we did the same for XMM and eROSITA).
• You can look through the XMM spectrum generation functions for inspiration and things to look out for.

The first tangible step for this task would be to create a 'spec' section in generate.ciao, then create an internal function (so with a '_' at the beginning of the name) called _chandra_spec_cmds or something - that is the same setup we use for XMM, and if we write _chandra_spec_cmds properly, it will be useful both for global/core-excised spectra and (later on) annular spectra.

When you look at the XMM spectrum functions you can ignore all of the bits which are todo with generating spectra within 'region' (so with outer_radius = 'region') - that is a legacy option left over from an older version of XGA and will be removed.