Unsigned integer warnings

I noticed a series of warnings while working on the PAM MMF integration:

```
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:14:31: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
14 | for (int icol = 0; icol < n; icol++) {
| ~~~~~^~~
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:15:33: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
15 | for (int irow = 0; irow < n; irow++) {
--
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:35:39: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
35 | for (int irow = idiag+1; irow < n; irow++) {
| ~~~~~^~~
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:37:39: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
37 | for (int icol = idiag; icol < n; icol++) {
| ~~~~~^~~
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:40:35: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
40 | for (int icol = 0; icol < n; icol++) {
| ~~~~~^~~
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:50:40: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
50 | for (int icol = irow+1; icol < n; icol++) {
| ~~~~~^~~
/global/u1/w/whannah/E3SM/E3SM_SRC4/externals/YAKL/src/extensions/intrinsics/YAKL_intrinsics_matinv.h:53:35: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
53 | for (int icol = 0; icol < n; icol++) {
| ~~~~~^~~
```