Minor fixes for v4.1

[gbarter]

Feature or improvement description

Two small bug fixes:

• gfortran on windows in the conda environment is finicky that IMPLICIT NONE come before any ATTRIBUTE lines. This is currently fixed in the conda build with a local patch but should be part of the source code.
• openfast_io functions when FAST_directory is not set results in errors since the default value is None (instead of an empty string). I kept the None default value, but added a check for it.

Not sure if this PR should be directed to the dev branch, or the rc4.1.1 branch.

Related issue, if one exists

Impacted areas of the software

Additional supporting information

Test results, if applicable

[deslaughter]

@gbarter That's a really strange bug with the implicit none in MinGW. Having implicit none at the top of the module, is sufficient, so could you remove it from the functions? I think that will be a cleaner solution that putting it before the attribute lines.

[gbarter]

Thanks, Derek. That seems to have worked. Relevant PR on the conda feedstock is here.

[mayankchetan]

Looks good to me, thanks for the fix @gbarter!