Add ch7, covariant tests

[sritchie]

This PR addresses issues noticed by @phasetr in ch7 of FDG. I also noticed a number of tests in covariant_derivative.scm that I had missed; porting those exposed a bug in the text, which is fixed as of this PR.

From the CHANGELOG:

• sicmutils.calculus.coordinate/generate moves to
  sicmutils.calculus.manifold/c:generate; this supports a bugfix where
  1-dimensional manifolds like R1-rect, aka the-real-line, return a
  coordinate prototype of a single element like t instead of a structure
  with a single entry, like (up t). Thanks to @phasetr for the bug report
  that led to this fix, and @gjs for finding and fixing the bug.

• same.ish/Approximate implemented for sicmutils.structure/Structure,
  allowing ish? comparison of up and down structures with approximate
  entries. Require sicmutils.generator for this feature. (NOTE: because
  protocols are implemented for the LEFT argument, (ish? <vector> (down ...)) will still return true if the values are approximately equal, even
  though a <vector> is technically an up and should NOT equal a down. Do
  an explicit conversion to up using sicmutils.structure/vector->up if
  this distinction is important.)

• same.ish/Approximate now defers to sicmutils.value/= for equality
  between Symbol and other types. This lets ish? handle equality between
  symbols like 'x and literal expressions that happen to wrap a single
  symbol.

• Cartan->Cartan-over-map now does NOT compose (differential map) with its
  internal Cartan forms. This fixed a bug in a code listing in section 7.3 of
  FDG.

• Section 7.3 of FDG implemented as tests in sicmutils.fdg.ch7-test.

• Many new tests and explorations ported over from covariant-derivative.scm.
  These live in sicmutils.calculus.covariant-test.

• timeout exceptions resulting from full GCD are now caught in tests using
  sicmutils.simplify/hermetic-simplify-fixture. Previously, setting a low
  timeout where simplification failed would catch and move on in normal work,
  but fail in tests where fixtures were applied.

[sritchie]

@phasetr, I will write more later, but I spoke with Sussman and you found a legit bug in the code from the book!

The fix is in the definition of g. Basically ((differential gamma) d:dt) should just be d:dt, since (differential gamma) is already applied to the d:dt argument inside ofCartan->Cartan-over-map:

https://github.com/sicmutils/sicmutils/blob/master/src/sicmutils/calculus/covariant.cljc#L228

Same fix for the scheme code in the book. Here is the correct g, and the beginnings of some tests that lock this in:

https://github.com/sicmutils/sicmutils/pull/381/files#diff-6731d08ad0eb5e693e47f0bebee3b1095fee2b86b99ff1606500aea548b53bbeR99-R103

[sritchie]

@phasetr ACTUALLY after another round with Sussman, and now that I understand the problem better, this PR will fix the library so that the code from the book works as is. That conceptually makes more sense. Hold tight until I get more tests ported and get this merged!

[codecov-commenter]

Codecov Report

Merging #381 (9da4158) into master (f406f69) will increase coverage by 0.53%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
+ Coverage   84.07%   84.61%   +0.53%     
==========================================
  Files          97       97              
  Lines       12876    12886      +10     
  Branches      688      691       +3     
==========================================
+ Hits        10826    10903      +77     
+ Misses       1362     1292      -70     
- Partials      688      691       +3     

Impacted Files	Coverage Δ
src/sicmutils/calculus/coordinate.cljc	86.04% <ø> (+0.94%)	⬆️
src/sicmutils/calculus/manifold.cljc	76.74% <77.77%> (+0.93%)	⬆️
src/sicmutils/calculus/basis.cljc	84.84% <100.00%> (ø)
src/sicmutils/calculus/covariant.cljc	85.46% <100.00%> (+16.53%)	⬆️
src/sicmutils/simplify.cljc	97.72% <100.00%> (+2.34%)	⬆️
src/sicmutils/numsymb.cljc	93.20% <0.00%> (+0.48%)	⬆️
src/sicmutils/rational_function.cljc	79.88% <0.00%> (+0.56%)	⬆️
src/sicmutils/calculus/indexed.cljc	79.14% <0.00%> (+0.94%)	⬆️
src/sicmutils/numerical/ode.cljc	94.38% <0.00%> (+1.12%)	⬆️
... and 4 more

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