wt.data.join does not work if any individual piece being joined is a single point

[DLafayetteII]

test.txt

I was confident I had done this successfully ~ a year ago, but this fails in WrightTools 3.4.4 and 3.4.6.
Guessing something the slice inside the transpose:

`
File "C:\Users\David Lafayette\miniconda3\lib\site-packages\WrightTools\data_join.py", line 262, in join
combine(data, out, channel_name, new_idx, transpose, slice_)

File "C:\Users\David Lafayette\miniconda3\lib\site-packages\WrightTools\data_join.py", line 199, in combine
vals[valid_index] = old[:].transpose(transpose)[slice_]

ValueError: axes don't match array
`

[DLafayetteII]

The issue arises from lines 245 - 250 of _join.py. np.ndim() of the 1 point variable returns a dimensionality of zero (np.ndim(d1.freq.points) = 0). Therefore, nothing is appended to the transpose, and an empty transpose (transpose = []) is passed to the combine method (If instead transpose = [0], the combine and join methods work as intended).

[DLafayetteII]

I'll propose a fix for the issue but I'm not sure I understand the initial intention of what should be done with 'scalar' (single-value) variables. There is a comment on line 248 about the transpose being unnecessary for scalar variables, but we pass the transpose to the combine method regardless of whether the variable is scalar.