fix #4216: str and latex representations for Bound variables

[Spaak]

Fixes #4216.

This:

import pymc3 as pm

with pm.Model() as model:
  bound_var = pm.Bound(pm.Normal, lower=1.0)('bound_var', mu=0, sigma=10)
  print(str(bound_var))

will now give this:

bound_var ~ Bound(lower=1.0, upper=None)-Normal(mu=0.0, sigma=10.0)

[codecov]

Codecov Report

Merging #4217 (87157bc) into master (3f14968) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4217   +/-   ##
=======================================
  Coverage   88.91%   88.92%           
=======================================
  Files          89       89           
  Lines       14429    14441   +12     
=======================================
+ Hits        12829    12841   +12     
  Misses       1600     1600           

Impacted Files	Coverage Δ
pymc3/distributions/bound.py	92.36% <100.00%> (+0.76%)	⬆️
pymc3/util.py	94.38% <100.00%> (ø)

[michaelosthege]

LGTM !
Feel free to mention this fix (linking to this PR) in the corresponding section of the RELEASE-NOTES.md

[Spaak]

@michaelosthege Thanks, and sorry for the slow reply. I've now added these (and the general __str__ overhaul for which this is a fix) to the release notes as PR #4227 .