WP 34S and 31S bugs and fixes

[Bit]

I've committed a patch to make the three options discussed here independently configurable: whether EEX is allowed in fraction mode at all, whether it exits fraction mode, and whether decimal fractions exit fraction mode. For the sake of completeness, it's also configurable whether entering a fraction automatically switches to fraction mode, and whether entering a mixed/proper fraction vs. a simple/improper fraction automatically switches to the corresponding type of fraction mode.

At the same time I've rearranged the code related to INCLUDE_EEX_PI, made it configurable whether it depends on flag L, removed some redundancy and fixed two small problems. One was triggered by invalid input in the command line (incomplete fraction), the other by a command line that was full (very long negative fraction).

Walter, Pauli and Marcus determine what goes into the official WP 34S so by default all new features are inactive and everything continues to work exactly as before until they decide otherwise.

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This issue seems to stir up emotions in a way I didn't expect but the discussion has made it clear to me that rational reasons to disable EEX can't be found. Marcus had a good idea and Nigel also that are worth exploring IMO. So instead of simply unblocking EEX as I originally intended, I've made EEX and decimal fractions exit fraction mode in my custom builds. I'm not yet fully convinced that automatically exiting fraction mode will prove to be an improvement so I'm open to further discussing its merits and I invite everyone to give it a try and see how well it works in practice.

(Edit: Fix typo.)