RField: add vector.reserve() wherever we know the number of elements...

[silverweed]

…to push

Changes or fixes:

Adding a call to vector.reserve() to RField.cxx in (almost) all cases where we know the exact number of elements pushed. Saves a bunch of reallocation especially when the number of pushed elements is not strictly bounded (e.g. number of columns, number of subfields).

Checklist:

• [x ] tested changes locally
• [ ] updated the docs (if necessary) - not necessary

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Test Results

    10 files      10 suites   1d 23h 39m 16s :stopwatch:  2 635 tests  2 635 :white_check_mark: 0 :zzz: 0 :x: 24 868 runs  24 868 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 6918bf48.

:recycle: This comment has been updated with latest results.