Issue29 axivity variable sf

This PR fixes #29

As of release 0.2.7 (last week) the code assumed a steady number of samples per block based on sampling rate and file type. This, I now realise, is not a safe assumption for Axivity as it can have a variable sampling rate.

As a result too much data is being skipped when searching for the next block when sampling rate is lower than expected. In the opposite situation when the sampling rate is higher than expected it is not a problem because the code will keep iterating over the blocks until it has found the correct timestamp.

Based on test data I see the error can be around 0.6%. So, I updated the code with an extra safety margin by only skipping 97% of the anticipated data to be skipped. So, for now I assume that the error in sampling rate never gets more than 3% accumulated over time.

UPDATE: I have now revised this to follow the logic:

Code first tries fraction 0.97 (if start of data segment to read is at 100, it will skip the first 97 and start reading block by block from there)
If that fails because somehow when we arrive at 97 we see that we are already beyond timestamp 100 then it goes back to beginning of the file and tries fraction = fraction - 0.1
The previous step is repeated until fraction is less than 0.2 after which it goes the beginning of the file and uses the old and slow approach and gives a warning to the user that something may be wrong with the file.

Codecov Report

Merging #30 (ff3ecaf) into main (77aed86) will increase coverage by 0.06%. The diff coverage is 82.35%.

:exclamation: Current head ff3ecaf differs from pull request most recent head b2c0636. Consider uploading reports for the commit b2c0636 to get more accurate results

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@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   86.72%   86.78%   +0.06%     
==========================================
  Files           9        9              
  Lines         919      946      +27     
==========================================
+ Hits          797      821      +24     
- Misses        122      125       +3

Impacted Files	Coverage Δ
R/readAxivity.R	`85.71% <82.35%> (+0.29%)`	:arrow_up:

wadpac / GGIRread

Issue29 axivity variable sf #30

Codecov Report