Adding contain and iter methods.

[germa89]

This PR does two things:

• Add a scape to interp_status for the edge case there is no parameters defined. Close Value Error if defining a parameter in the Begin level #1107

• Implement __contain__ and __iter__ methods. This way now we can do:
  Because of the __contain__:

>>> mapdl.parameters['TWO'] = 2.0
>>> "TWO" in mapdl.parameters
True
>>> "THREE" in mapdl.parameters
False

Because of the __iter__:

>>> for each in mapdl.parameters:
>>>     print(each)
"TWO"

Simple things, that I believe they will be useful.

[codecov]

Codecov Report

Merging #1108 (d47a9a6) into main (95ce5cf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1108      +/-   ##
==========================================
- Coverage   73.31%   73.31%   -0.01%     
==========================================
  Files          43       43              
  Lines        6428     6434       +6     
==========================================
+ Hits         4713     4717       +4     
- Misses       1715     1717       +2     

[akaszynski]

Agree with the implementation. We've made parameters a dict-like, and as such we should include these dunder methods as well.

[germa89]

I wonder if we shouldn't have just inheritate from dict. I recall you were happy to do that for the "info" module, but not sure about this case.