Fix incorrect MRT calculation for some epws

[Tom-Kingstone]

NOTE: Depends on

Issues addressed by this PR

Closes #232

Changes made in accordance with linked issue.

If sun exposure is null, then use sky exposure instead when calculating shaded and unshaded MRT.

Test files

Verify that MRT is being calculated properly (@tg359 has an epw that caused this issue before)

Changelog

Additional comments

[Tom-Kingstone]

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[tg359]

Based on a functionality check with the weatherfile that was showing issues (and another which wasn't) nothing seems to have broken and the changes make sense, so I approve them.

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