Make PLY parser more tolerant towards files not adhering to standard

[kunaltyagi]

Better error messages, better loops, less indentation.

Supersedes and closes #3527 . Fixes #3487. Closes #3353

[kunaltyagi]

Test 110: test_non_linear is flaky on my system

[kunaltyagi]

so i think pcl can improve the fault tolerance. corlor or position loss may replace by default

#3353 (comment) #3527 (comment)

With better error messages, it'll be easier to understand what's wrong. Making PCL more fault tolerant will require:

• Zero as default: depends on UI. 0, ie: black (for color) or origin (for points) as missing doesn't make universal sense. Needs change in the API to accommodate "missing value"
• Skip (and warn) items with missing properties: if it reads next element properties for current element that's cause for trouble. Needs setting in API to make it a conscious choice by the user

PCL is not a UI tool and is also used in cases where there's near zero user feedback. As such, it needs to ensure that the input is perfect (or bail out with an error) and none of the above options can achieve that (without more extensive changes). Missing fields are a sign of a larger error and dealing with them will only cause more issues for the rest of the input file. A best effort reader will require configs and options and that'll take more effort.

TLDR: Not against the proposal for a best-effort reader, just against that in the current provided API.

[taketwo]

OK, that's convincing, thanks. Please squash.