Fix transpose to work on single pol data, with test

[dentalfloss1]

The diff is actually correct this time! Fixes transpose to work on single polarization data. Includes working with mono wav files in the tutorial. Added a test for these kinds of datasets.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0388e5f) 65.39% compared to head (fbf27be) 65.39%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #207 +/- ## ======================================= Coverage 65.39% 65.39% ======================================= Files 67 67 Lines 5840 5840 ======================================= Hits 3819 3819 Misses 2021 2021 ```

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[league]

The successful test speaks for itself! But I'm trying to understand what was broken about it in the first place, and I'm wondering if it can be fixed without special-casing K==1. Was the issue maybe because in_inds_str contained a reference to k when it's mono? Then k wouldn't exist until inside the loop, and in this fix, k is declared without a loop (because the loop would be singleton anyway). Would it be the same to produce something like:

func_str += "enum { K = " + std::to_string(K) + " };\n";
func_str += "int k=0;\n";
func_str +=
    "in_type ivals = in(" + in_inds_str + ");\n"
    "#pragma unroll\n"
    "for( ; k<K; ++k ) {\n"
    "    out(" + out_inds_str + ") = ivals[k];\n"
    "}\n";      

Does that work-alike when K==1 ? Maybe I'm confused about why it wasn't working in the first place?

[league]

I was able to try what I thought might unify the K==1 case with the general case. But it doesn't work! Passing the test is the main thing, so I think this fix can be merged!

When it's passing the tests, here is an example of generated code when K==1:

enum { K = 1 };
int k=0;
out_type ovals = in(k, i0, i1);
out(i0, i1) = ovals;

compared to when K==2:

enum { K = 2 };
in_type ivals = in(i2, i1);
#pragma unroll
for( int k=0; k<K; ++k ) {
    out(k, i1, i2) = ivals[k];
}

Oh, one little simplification incoming in a moment…