2 small suggestions related to save()

[Kris Thielemans]

Hi

I was confused by the usage of default_save_options. I assigned a value to
the variable, as opposed to calling the function. This is of course clearly
documented ("help default_save_options" tells you), but nevertheless I only
found out when trying to assign a value to save_header_format_string. Doing
that generates a clear warning about function vs variable. I wonder
therefore if it wouldn't be helpful to issue the same warning for
default_save_options. This would be easy to do (just add it to
former_built_in_variables in pt-assign.cc). 

Another minor suggestion is to mention save_header_format_string in the
documentation of save() (e.g. as "see also"). I only found out about it by
checking the code.

Many thanks

Kris