gnss_poser does not parse MGRS zone correctly

[VRichardJP]

Checklist

• I've read the contribution guidelines.
• I've searched other issues and no duplicate issues were found.
• I'm convinced that this is not my fault but a bug.

Description

In GNSSStat, there is no support for MGRS zone (e.g. "53SPU"). There is a int zone which can be used for utm zone, but MGRS would need a string.

Yet, in the UTM2MGRS function, the MGRS zone (e.g. "53SPU") is converted to int using stod:

autoware.universe/sensing/gnss_poser/include/gnss_poser/convert.hpp

Line 146 in d5db3ac

mgrs.zone = std::stod(mgrs_code.substr(0, GZD_ID_size));

There are 2 problems with this piece of code:

1. since zone is a int, it is strange to use stod instead not stoi
2. std::stod("53SPU") returns 53.0, which looses the second part of the code SPU

Although the value mgrs.zone is not used elsewhere in gnss_poser, it could cause some issue in the future.

Expected behavior

N/A

Actual behavior

N/A

Steps to reproduce

N/A

Versions

No response

Possible causes

No response

Additional context

No response

[YamatoAndo]

@VRichardJP Thank you for the report.
You are right, there are problems with this code.
Can you please issue a PR?