fix template disambiguation syntax on MSVC

[PerretB]

MSCV currently emits a syntax error on line (TBB with SIMD branch)

xtensor/include/xtensor/xassign.hpp

Line 1252 in 8c0a484

res_stepper.template store_simd(fct_stepper.template step_simd<value_type>());

Adds a simple syntax helper for MSVC (not sure what the standard says here).

res_stepper.template store_simd<>(fct_stepper.template step_simd<value_type>()); 

FIXES #2736 (note that compiling with /Zc:lambda on MSVC is currently required to avoid this compiler bug https://developercommunity.visualstudio.com/t/lambda-fails-to-implicitly-capture-constexpr-value/610504 )

[spectre-ns]

I've been wondering how to fix this. The default lambda parser is definitely not compliant with constexpr captures. How did you know that adding <> would fix this?

[PerretB]

I had a vague memory of having encountered a similar problem with the template keyword before, but I can't say I knew it would work :D

[spectre-ns]

Thanks! This will help my code base.

@JohanMabille please review! 🙂