Add documentation about new is_available feature category

{dump}, {fix}, {group}, {region}, and {variable} whether an entry

with the provided name or id is defined.

The {is_available()} function allows to query whether a specific

optional feature is available, i.e. compiled in. This currently

works for the following categories: {command}, {compute}, {fix},

and {pair_style}. For all categories except {command} also appending

active suffixes is tried before reporting failure.

The {is_available(category,name)} function allows to query whether

a specific optional feature is available, i.e. compiled in.

This currently works for the following categories: {command},

{compute}, {fix}, {pair_style} and {feature}. For all categories

except {command} and {feature} also appending active suffixes is

tried before reporting failure.

The {feature} category is used to check the availability of compiled in

features such as GZIP support, PNG support, JPEG support, FFMPEG support,

and C++ exceptions for error handling. Corresponding values for name are

{gzip}, {png}, {jpeg}, {ffmpeg} and {exceptions}.

This enables writing input scripts which only dump using a given format if

the compiled binary supports it.

if "$(is_available(feature,png))" then "print 'PNG supported'" else "print 'PNG not supported'" :pre

if "$(is_available(feature,ffmpeg)" then "dump 3 all movie 25 movie.mp4 type type zoom 1.6 adiam 1.0" :pre

:line