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()