compare, == ,and isapprox, verbose output, skip

[Affie]

Taking up ideas from #309

Dumping some ideas I wrote down a while ago:

• == equality
• ≈ /isapprox - Inexact equality
• compare - flexible
  • eg. compareDFG(a, b, skipfields=Symbol[]; eq = ==, by=identity)

[dehann]

FYI i have been making progress on comparing KDEs.

[dehann]

See AMP v0.1.4 for introduction of isapprox for beliefs (as requested).