New aggregates last minus first and high minus low

[texodus]

This PR is a rebased, modified and tested version of #1739. The major changes:

• The calculation Xn - X0 for these aggregate types used the double-promoting operator implementations for t_tscalar, (which is required by ExprTK), however the output column type for these aggregates was not promoted, leading to all 0/null values. This PR adds a new set of type-preserving methods add_typesafe and sub_typesafe that can be used safely with the new aggregates.
• first_last_helper method was being called twice, when internally this method does a column-scan and calculates both values for each pass. This PR modifies the signature of first_last_helper to just return the tuple which can be decomposed by the caller, eliminating the extraneous iteration. This aggregate should really use the build_aggregate<> pattern like AGGTYPE_HIGH_WATER_MARK does currently..
• Adds tests, fixed a typo, removed spurious .gitignore changes.

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[finos-cla-bot]

Thank you for your contribution and Welcome to our Open Source Community!

To make sure your pull request is accepted successfully, we ask all our open source contributors to sign a Contributor License Agreement; having reviewed our contributor list, we require a CLA for the following people : (@shinny-yangyang).

In order to sign a CLA with FINOS, just submit a Pull Request with a simple change to this file (adding an empty line, or some random text at the bottom); this will trigger the EasyCLA bot, which will post a comment to the Pull Request stating whether all PR contributors are covered by FINOS CLA; if not covered, the bot will post instructions on how to sign the CLA.

Thanks once again for your contribution. Let us work with you to make the CLA process quick, easy and efficient so we can move forward with reviewing and accepting your pull request. Feel free to email [email protected] for any questions.

cc @maoo @TheJuanAndOnly99 @mcleo-d

[shinny-yangyang]

@texodus Thank you very much for your help. I still confused that is it necessary to add every aggtype to t_config::setup() (line 186 of config.cpp)? If so, why AGGTYPE_HIGH_WATERMARK and AGGTYPE_LOW_WATERMARK are not there?