WIP first version of tps speedup

[jefferis]

• separate computation of tps xform from applying it
• memoise as well so it only runs once
• much faster for larger/unstable tps pointsets

[codecov[bot]]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (f2f35ab) 76.88% compared to head (da771b0) 76.80%. Report is 2 commits behind head on master.

Files	Patch %	Lines
R/xform.R	0.00%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #515 +/- ## ========================================== - Coverage 76.88% 76.80% -0.08% ========================================== Files 48 48 Lines 5935 5941 +6 ========================================== Hits 4563 4563 - Misses 1372 1378 +6 ```

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