tpl/math: Allow variadic math functions to take slice args, add math.Product, math.Sum

[bep]

• Update math.Min and math.Max to allow 1 or more args, either scalar or slice, or combination of the two
• Add math.Sum (allow 1 or more args, either scalar or slice, or combination of the two)
• Add math.Product (allow 1 or more args, either scalar or slice, or combination of the two)

Fixes #11030

[jmooring]

These throw the "must provide at least two numbers" error:

{{ math.Sum 1 }}
{{ math.Sum (slice 1) }}
{{ math.Product 1 }}
{{ math.Product (slice 1) }}
{{ math.Min 1 }}
{{ math.Min (slice 1) }}
{{ math.Max 1 }}
{{ math.Max (slice 1) }}

[bep]

@jmooring I have pushed an update that removes that validation, meaning that e.g.

{{ math.Sum (slice) => 0 }}

Which I guess makes sense.

[jmooring]

1) These work as desired (1 arg... an empty slice):

{{ math.Sum slice }}     --> 0
{{ math.Product slice }} --> 0
{{ math.Min slice }}     --> 0
{{ math.Max slice }}     --> 0

2) But I think these should throw an error (no args):

{{ math.Sum }}      --> 0
{{ math.Product }}  --> 0
{{ math.Min }}      --> 0
{{ math.Max }}      --> 0

---

The use case for the first example above is:

{{ $awards := slice }}
{{ range where site.RegularPages "Type" "films" }}
  {{ $awards = $awards | append .Params.awards }}
{{ end }}

{{ math.Sum $awards }} 
{{ math.Min $awards }} 
{{ math.Max $awards }} 

[bep]

How does my last attempt ... work?

[jmooring]

Nice. Thank you.

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