Fix printing of empty mixed_units

[Enchufa2]

Subsetting worked ok. It was the print and str methods which were buggy for the cases described in #248.

[codecov[bot]]

Codecov Report

Merging #256 (2ec8559) into master (cc1911f) will decrease coverage by 0.10%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   94.20%   94.09%   -0.11%     
==========================================
  Files          19       19              
  Lines         914      914              
==========================================
- Hits          861      860       -1     
- Misses         53       54       +1     

Impacted Files	Coverage Δ
R/mixed.R	97.67% <90.00%> (-2.33%)	:arrow_down:

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[Enchufa2]

Also, I found that the c.mixed_units method was defined but not exported. Fixed here too.

[Enchufa2]

@edzer Kind reminder.