Performance Optimizations and additional Features in invesdwin-util fork

[subes]

Hi,

thanks a lot for this great expression parser. I did some magic on it here: https://github.com/subes/invesdwin-util/tree/master/invesdwin-util/src/main/java/de/invesdwin/util/math/expression

This is essentially a fork that adds the following improvements:

• time series support via FDate and int indexed evaluation in addition to no-key variation as exists here
• [x] operator for accessing previous values via index/expression (IPreviousKeyFunction needs to be provided for this feature), works on variables and functions by just modifying the key for evaluation
• prefer final and immutability where possible
• remove scope and instead allow overriding getVariable and getFunction methods
• redirect functions to variables and vice versa where possible
• crosses under/crosses above/not operators
• efficient boolean expression processing including skipping unnecessary evaluations
• remove unneeded features from tokenizer and improve overall tokenizer performance

I felt the modifications were too much to create pull requests, so instead I did a major rewrite of most parts and published the results in invesdwin-util. I might find more improvements which I will also make public in that repo.

You might be able to pick some of those improvements and reintegrate them in parsii. Otherwise you can just close this issue since I just wanted to inform you about the performance improvements that are still available.

Best regards,
Edwin