[feat] Support parameter and variable access directly from the class body

[jjotero]

This PR implements the attribute access to variables and parameters directly from the class body. Prior to this PR, an attempt to access either variables or parameters from the class body resulted in a NameError since none of these built-ins are stored in the public namespace from the class. This PR not only implements the access to these items, but also provides a more robust error handling in case of erroneous user input:

• Accessing a required (undefined) variable in the class body will raise a ValueError:

class Foo(rfm.RegressionMixin):
    v = variable(str)
    x = f'{v}' # This will raise an error because `v` has no value

• Accessing a defined variable will return its default value:

class Foo(rfm.RegressionMixin):
    v = variable(int, value=4)
    x = v + 1 # this sets x to 5

• Accessing a parameter directly from the class body is disallowed.

class Foo(rfm.RegressionMixin):
    p = parameter([1, 2, 3])
    x = f'{p}' # This raises an error. Here `P` is a TestParam object, not a test parameter.

[codecov-io]

Codecov Report

Merging #1806 (0e31e60) into master (9d02ba4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1806      +/-   ##
==========================================
+ Coverage   87.57%   87.59%   +0.02%     
==========================================
  Files          49       49              
  Lines        7922     7935      +13     
==========================================
+ Hits         6938     6951      +13     
  Misses        984      984              

Impacted Files	Coverage Δ
reframe/core/meta.py	100.00% <100.00%> (ø)
reframe/core/pipeline.py	91.72% <100.00%> (ø)

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[vkarak]

lgtm