Get gmse_apply to find custom functions in old_list

[bradduthie]

As noticed by @jeremycusack -- which we both thought was a bug -- the gmse_apply function requires re-stating custom functions as arguments even when such arguments are available in the old_list argument. It shouldn't be difficult to get gmse_apply to recognise when it has a custom argument from an old list, then use that argument (overriding the default that is assumed when the argument is not specified).