Allow insert!ion of AbstractRuleNodes

[ReubenJ]

Currently, restricting to concrete RuleNodes restricts the user from inserting a Hole, for example.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 46.10%. Comparing base (aca61f9) to head (0f92127).

Files	Patch %	Lines
src/nodelocation.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   46.10%   46.10%           
=======================================
  Files           8        8           
  Lines         462      462           
=======================================
  Hits          213      213           
  Misses        249      249           

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[THinnerichs]

As @nicolaefilat said: Your change is valid.
However, with the current implementation you cannot substitute the entire tree with a hole, if I am not mistaken.
This would be possible when constricting to RuleNodes specifically.

[ReubenJ]

This is missing a test, which, if we had it, would show that Holes do not have an ind field. Add a new method for Holes instead.

[ReubenJ]

Closing in favor of releasing on v0.2.x branch