Revert throwing an error for non-thin intervals

[OlivierHnt]

Revert PR #613 which broke code compatibility with 0.22.

At the time the PR #613 was made, it was wrongly assumed that no-one was relying on ==(::Interval, ::Number) to work, since it has been in the past. This may not be the case, e.g. see lairez/ExactPredicates.jl#25.

So the behaviour is as follows:

• iszero, isone and isinteger are semantically equivalent to isthinzero, isthinone and isthininteger respectively. Their behaviour is in agreement to Base docstrings
• x == y between an interval x and a number y is permitted and is semantically equivalent to isthin(x, y).

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (ba420dc) 82.74% compared to head (5c8b757) 82.60%.

Files	Patch %	Lines
src/intervals/interval_operations/boolean.jl	0.00%	1 Missing ⚠️
src/intervals/real_interface.jl	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
- Coverage   82.74%   82.60%   -0.14%     
==========================================
  Files          25       25              
  Lines        2167     2144      -23     
==========================================
- Hits         1793     1771      -22     
+ Misses        374      373       -1     

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