Updates for nwdFracLut test

[mauzey1]

Closes #539
Resolves #535

This PR fixes the issues that I discussed in #535 (comment) that were causing the nwdFracLut test to fail.

[taylor13]

Am I correct that you went with option 2: Look at all string dimensions and define "strlen" to be maximum length of all values? If so and if

1. it passes the test suite, and
2. it works for the case causing problems
   Then I think we can approve.

[mauzey1]

@taylor13 The NetCDF dimension "strlen" will get defined with the greatest string length of all string axes and singletons if a string axis is found in the function cmor_define_dimensions. If it is not defined in that function, then "strlen" will be defined with the greatest string length of all string singletons in the function create_singleton_dimensions.

[taylor13]

sounds good to me.