Prevent untyped calculates from being used as calculation triggers

[lognaturel]

Closes #486

@gushil I took this on to hopefully give you a little extra time for #485. I think you're best positioned to see if there's a way to make it a little easier to work with. I should hopefully also have a bit of time for it tomorrow.

Why is this the best possible solution? Were any other approaches considered?

I didn't consider any alternatives. This extends the existing logic to treat any question with a calculate type as ineligible to be a calculation trigger.

What are the regression risks?

The only change is to the logic that for a given question suppresses body tags and produces an error if it's used as a trigger. The worst case would be if this adds cases that are actually suppose to lead to body elements

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• [x] included test cases for core behavior and edge cases in tests_v1
• [x] run nosetests and verified all tests pass
• [x] run black pyxform to format code
• [x] verified that any code or assets from external sources are properly credited in comments

[codecov-io]

Codecov Report

Merging #489 (81e293c) into master (a584303) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #489   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files          25       25           
  Lines        3651     3651           
  Branches      849      849           
=======================================
  Hits         3055     3055           
  Misses        452      452           
  Partials      144      144           

Impacted Files	Coverage Δ
pyxform/question.py	93.30% <100.00%> (ø)

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[gushil]

Thanks @lognaturel