fix issue #15: non-character argument

[cryolune]

Description

Added as.character() to solve the error caused by strsplit function.

Motivation and Context

Error: Problem with `mutate()` column `pfts`.
ℹ `pfts = strsplit(pfts, "|", fixed = TRUE)`.
x non-character argument

This change fixes issue #15

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.