v3.5.0 - Unicode handling improvements

Major improvements:

• The library now scans for whitespace in a fully unicode-aware manner. It warns when whitespace other than that officially allowed in the STAR specification is found (' ', '\t', '\r', '\n', '\v') but it still parses it without issues. If raise_parse_warnings is set, an exception is thrown.

Minor improvements:

• pynmrstar.Loop.category is converted to a property to be able to validate it when set. Previously, invalid loop categories (tag prefixes) could be provided.

Potentially breaking changes:

• pynmrstar.definitions.WHITESPACE has been removed. This allowed users to set custom characters that they considered whitespace. This functionality was never encouraged. Instead, the standard STAR whitespace characters are considered official whitespace characters, but anything in Unicode considered whitespace is also handled properly.