Replace "derf()" with standard "erf()" in SBM MP scheme (#1183)

[joshbenz1]

TYPE: bug fix

KEYWORDS: PGI, compile, mp_SBM_polar_radar, phys, erf, derf

SOURCE: Bart Brashers (Ramboll)

DESCRIPTION OF CHANGES: A fortran function (DErF) was being used in the module_mp_SBM_polar_radar file. This was causing a compiling fail for PGI (tested with versions
19.10.0 on user's machine and 19.3 on Cheyenne). According to this article:
https://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/DErF-Intrinsic.html
the DErF function is archaic. Modifying this function to ErF corrects the problem and allows the
PGI compiler to build the WRF executable from source.

LIST OF MODIFIED FILES:
M phys/module_mp_SBM_polar_radar.F

TESTS CONDUCTED:

Verified that code does not compile in original state, but does with the modified code when
using the PGI compiler.
Reggie should be OK.
RELEASE NOTE: A minor modification has been introduced into the SBM Polar Radar microphysics code to allow the WRF code to build with PGI compilers. The nonstandard derf() intrinsic has been replaced by the 2008 standard compliant erf() intrinsic.

Use this template to give a detailed message describing the change you want to make to the code.
If you are unclear on what should be written here, see https://github.com/wrf-model/WRF/wiki/Making-a-good-pull-request-message for more guidance

The title of this pull request should be a brief "purpose" for this change.

--- Delete this line and those above before hitting "Create pull request" ---

TYPE: choose one of [bug fix, enhancement, new feature, feature removed, no impact, text only]

KEYWORDS: approximately 3 to 6 words (more is always better) related to your commit, separated by commas

SOURCE: Either "developer's name (affiliation)" .XOR. "internal" for a WRF Dev committee member

DESCRIPTION OF CHANGES: One or more paragraphs describing problem, solution, and required changes.

ISSUE: For use when this PR closes an issue. For example, if this PR closes issue number 123
Fixes wrf-model#123

LIST OF MODIFIED FILES: list of changed files (use git diff --name-status master to get formatted list)

TESTS CONDUCTED: Explicitly state if a WTF and or other tests were run, or are pending. For more complicated changes please be explicit!

RELEASE NOTE: Include a stand-alone message suitable for the inclusion in the minor and annual releases. A publication citation is appropriate.