pull request template

[mlee03]

Describe the bug
Currently, FRE-NCTools does not have a pull request template that Github recognize. Changing the current template filename, pull_request.md, to PULL_REQUEST_TEMPLATE.md resolves the issue.

To Reproduce
Steps to reproduce the behavior

Expected behavior
Users should see required fields to fill out when opening a pull request in FRE-NCTools

System Environment
Describe the system environment, include:

• OS: [e.g. RHEL 7.2]
• Compiler(s): Type and version [e.g. Intel 19.1]
• netCDF Version: For both C and Fortran
• Configure options: Any additional flags, or macros passed to configure