[feat] Change time_limit attribute to a string-based format

[rsarm]

The current format to create TimerField objects is with a tuple (hours, minutes, seconds).
The PR changes the format to a string like '1d10h30m2s', where 'd', 'h', 'm' and 's' stand for days, hours, minutes and seconds. Simpler strings can be used as '1d' or '1h30m'.

Closes #1106

[pep8speaks]

Hello @rsarm, Thank you for updating!

Cheers! There are no PEP8 issues in this Pull Request!Do see the ReFrame Coding Style Guide

Comment last updated at 2020-02-21 12:28:30 UTC

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1a576b1). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1142   +/-   ##
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  Coverage          ?   91.78%           
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  Files             ?       81           
  Lines             ?    11391           
  Branches          ?        0           
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  Hits              ?    10455           
  Misses            ?      936           
  Partials          ?        0

Impacted Files	Coverage Δ
reframe/core/containers.py	100% <100%> (ø)
unittests/test_containers.py	96.72% <100%> (ø)

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[vkarak]

The old syntax should not be abolished, but deprecated instead. You will need also a unit test to check that a deprecation warning is raised. If we merge this as it is now, it will break several tests.

[vkarak]

Just only some final minor suggestions.

[vkarak]

@rsarm You'd need also to fix the conflicts before we merge.