[bugfix] Fix variable and parameter access as class attributes

[jjotero]

This PR fixes a bug that was causing variables and parameters set as class attributes to be silently overridden. Bug reproducer:

class Base(rfm.RegressionTest):
    ...
    p = parameter([1])
    v = variable(int, value=1)

Base.p = [2] # This PR will disallow updating parameters like this.
Base.v = 2

@rfm.simple_test
class Derived(Base):
    def __init__(self):
        print(self.p) # Prints 1
        print(self.v) # Prints 1. With this PR this prints 2.

This PR disables setting/updating parameters when accessed as class attributes. This is because such parameter update would break the internal iterators on the parameter space. If this feature is required, explicit insert_parameter or update_parameter class methods could be added.

This might be needed by #563

[vkarak]

Just a couple of minor comments.

[codecov-commenter]

Codecov Report

Merging #1988 (e52babc) into master (6a00e3b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1988      +/-   ##
==========================================
+ Coverage   87.59%   87.61%   +0.02%     
==========================================
  Files          50       50              
  Lines        8770     8788      +18     
==========================================
+ Hits         7682     7700      +18     
  Misses       1088     1088              

Impacted Files	Coverage Δ
reframe/core/meta.py	100.00% <100.00%> (ø)
reframe/core/variables.py	96.03% <100.00%> (-0.02%)	⬇️

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