Fix issues with formula interface

[mayer79]

Fixes problem with syntactically wrong column names:

• https://github.com/mayer79/missRanger/issues/19
• https://github.com/mayer79/missRanger/issues/51

Additionally, at least for the default formula ". ~ .", parsing data.frames with many thousands of columns would not fail anymore. This has partly fixed by catching the ". ~ ." case in the initial formula parsing, and switching to ranger()s "x/y" interface. This solves

• https://github.com/mayer79/missRanger/issues/50

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9453a1e) 95.16% compared to head (57eabbe) 97.35%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #52 +/- ## ========================================== + Coverage 95.16% 97.35% +2.19% ========================================== Files 4 4 Lines 186 189 +3 ========================================== + Hits 177 184 +7 + Misses 9 5 -4 ``` | [Files](https://app.codecov.io/gh/mayer79/missRanger/pull/52?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Mayer) | Coverage Δ | | |---|---|---| | [R/missRanger.R](https://app.codecov.io/gh/mayer79/missRanger/pull/52?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Mayer#diff-Ui9taXNzUmFuZ2VyLlI=) | `97.03% <100.00%> (+3.09%)` | :arrow_up: |

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