Inconsistency between find_percentile_coordinate and find_threshold_coordinate functions

[tjtg]

During review in #1719, it was identified that the find_percentile_coordinate and find_threshold_coordinate functions in improver/metadata/probabilistic.py work differently to each other. In particular, find_percentile_coordinate contains additional logic to handle more than one percentile coordinate.
There are also simple is_percentile and is_threshold functions which use the find coordinate functions to identify the kind of probabilistic data being processed.

From some repository history digging, it looks like the two functions were implemented at different times and in different locations, then refactored to live in the same file as each other. Related PRs and issues: #974, #838, #833 and #314.

Acceptance criteria:
Option 1: updated code, either consistent between the two functions or removing duplication between the functions
Option 2: updated documentation explaining why the differences exist