**harm spine parser does not handle rests

[alexandermorgan]

music21 version

8.3.0

Problem summary

The humdrum **harm parser does not parse rest tokens in the **harm spine despite them being part of the **harm syntax as detailed here: https://www.humdrum.org/rep/harm/index.html

Steps to reproduce
Importing a kern file with a rest in the **harm spine raises an error.

# This raises an error:

**harm	**kern
1r	1r
*-	*-

# Omitting the duration of the rest in the **harm spine also raises an error:

**harm	**kern
r	1r
*-	*-

# Using a null token period correctly does not raise an error. This one is fine:

**harm	**kern
.	1r
*-	*-

Expected vs. actual behavior

Rest tokens should be allowed in **harm spines since they're part of the syntax. If there's no music21 representation of this it would be fine to treat them the same as a "." null token.
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