[bugfix] Allow proper assignment of time_limit from command line

[vkarak]

This PR fixes the same problem for all other TimerField variables.

To allow setting these fields properly from the command line, we get rid of the TimerField and replace it with the augmented type Duration, which is similar in concept with the Bool type. This type allows conversion from floats, int and strings and applies essentially the logic of the former TimerField. Since for this field we want to allow conversions also when it's set from inside the test, e.g., time_limit = '10s', we add a new argument allow_implicit to the TypedField constructor (also exposed through the variable builtin), which make the field try implicit conversions before assigning the value to the test attribute. Therefore a variable declared as x = variable(int, allow_implicit=True) can be set with either x = 1 or x = '1'.

Closes #2815.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (f1b0b15) 86.76% compared to head (6835b6d) 86.79%.

❗ Current head 6835b6d differs from pull request most recent head cc83890. Consider uploading reports for the commit cc83890 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2823      +/-   ##
==========================================
+ Coverage   86.76%   86.79%   +0.03%     
==========================================
  Files          60       60              
  Lines       11351    11348       -3     
==========================================
+ Hits         9849     9850       +1     
+ Misses       1502     1498       -4     

Impacted Files	Coverage Δ
reframe/core/fields.py	97.19% <100.00%> (-0.37%)	⬇️
reframe/core/pipeline.py	93.88% <100.00%> (ø)
reframe/core/schedulers/__init__.py	98.38% <100.00%> (-0.01%)	⬇️
reframe/utility/typecheck.py	97.35% <100.00%> (+2.49%)	⬆️

... and 1 file with indirect coverage changes

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