0.9.31

• cleanup numpy imports to avoid deprecated functions, add financial functions from numpy_financial module, if installed.
• prefer 'user_symbols' when initializing Interpreter, but still support 'usersyms' argument. Will deprecate and remove eventually.
• add support of optional (off-by default) "nested symbol table", based on a Group object that can be accessed from Python either with dict syntax (ie aeval.symtable['x']) or with object attribute access (aeval.symtable.x). In addition, symbols can be found by looking first in the top-level symbol table and then in sub-Groups named in symtable._searchgroups, allowing for a more hierarchical, nested symbol table. This should be considered experimental and is off by default. The make_symbol_table() function gains a nested=False argument to control whether to use this option.
• update tests to run most tests with symbol tables of dict and nested group type.
• general code and testing cleanup.