Replace nonstandard isnan() call with value input checks instead

[islas]

TYPE: bug fix

KEYWORDS: compilation, nonstandard

SOURCE: internal

DESCRIPTION OF CHANGES:
Problem:
PR #1944 introduced a nonstandard function call isnan() which breaks compilation for compilers which don't support this extension.

Solution:
A potential alternative is to use the ieee_is_nan() call from ieee_arithmetic. As there is conditional compilation of this module which requires Fortran 2003 standard, an easier alternative is just to check the input values before calling acos()

[weiwangncar]

The regression test results:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None

[mgduda]

As a final comment, I wonder whether the original code is correct? Would setting angle = pi be better when tmp1 / tmp2 is less than -1?

[islas]

As a final comment, I wonder whether the original code is correct? Would setting angle = pi be better when tmp1 / tmp2 is less than -1?

I'm unsure. That does make sense, but I was hesitant to modify the logic in a way that deviates from the original. Since the final result is a logical, there are probably more implications [than I understand] based on what a close-to or actual out of bounds evaluation should be.

[mgduda]

Agreed that we shouldn't try to "correct" the computation, as we may be missing some important details.

As a final comment, I'd suggest trying to match the whitespace convention used in the rest of the find_turb: four spaces of indentation, and no space between the parentheses in conditional statements.

[islas]

trying to match the whitespace convention used in the rest of the find_turb: four spaces of indentation, and no space between the parentheses in conditional statements.

Fixed, though I do wish we had a format checker, as a significant portion of the file's code (and the original logic that this PR replaces) don't follow this.