Feat: threshold checker interval string

[liyier90]

Closes https://github.com/aimakerspace/PeekingDuck-Private/issues/83

Changes:

• Uses a string to represent the interval of valid values.
• Checks formatting of the interval string using regex (see details below).

The interval string must match ^[\[\(]\s*[-+]?(inf|\d*\.?\d+)\s*,\s*[-+]?(inf|\d*\.?\d+)\s*[\]\)]$:

• ^: Start of line
• [\[\(]: Either [ or (
• \s*: Any whitespace characters between 0 and unlimited times
• [-+]?: - or + between 0 and 1 time
• (inf|\d*\.?\d+): Either inf or a decimal number where having a digit before the period is optional, e.g., .9. Ref comment by Chandranshu
• \s*: Whitespace
• ,: Separator
• \s*: Whitespace
• [-+]?(inf|\d*\.?\d+): Optional - or + with either inf or a decimal number
• \s*: Whitespace
• [\]\)]: Either ] or )
• $: End of line

Since we are trying to use a mathematical representation of the interval, I have opted to use -inf and +inf for correctness.

[ongtw]

Some comments for consideration.

[ongtw]

Only one issue outstanding.

[liyier90]

Set an upper limit to protobuf version. Similar problems have been reported in other projects. issue

[ongtw]

Looks good to merge.