Frozen networkx

For SMC, uses frozen versions of networkx.

date and time functions

new functions branch

• but with 'and' and 'or' functions removed from astnodes.py (where they are implemented by 'all' and 'any' respectively), to exellib.py. This is to facilitate an implementation which replicates excel's behaviour when 'and' or 'or' functions are applied to ranges which contain any empty cells.

• also, cells with constant values (i.e. no formula) have that value preserved through spreadsheet resets.

• also, date and time related functions have been added.