Build with nagfor(7.0) is fail because of "isnan"

[sakamoti]

There are many isnan functions are used in test_mean.f90, test_moment.f90, test_var.f90 and test_varn.f90 files. But isnan is not a fortran standard. So, NAG fortran compiler (nagfor) fail to build these files. What about using ieee_is_nan instead of isnan?

[jvdp1]

Thank you for testing stdlib.
ieee_is_nan could be used indeed. ieee_arithmetic is already used to generate NaN in mean(), moment(), and var() functions.

If you have time, could you modify the tests and submit a PR, please?

There has been several discussions around NaN and its implementation (e.g., #147). A more general solution is needed. Meanwhile, using ieee_is_nan should be fine, and a better solution than using compiler-dependent is_nan.

[sakamoti]

Thank you for merging pull request #160 . Close this issue.