Hbmcmc fails for multiple sites if a site is missing data at the beginning of the month being run

[rt17603]

Original report by Rebecca Ward (Bitbucket: [Rebecca Ward](https://bitbucket.org/Rebecca Ward), GitHub: rebeccawardbristol).

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When using multiple sites in hbmcmc the code fails when using footprints_data_merge if one of the sites is missing data at the beginning of the month being run.

In the past people have found their own ways to bypass this issue such as by manually removing the site with the missing data from the ini file. It would be good to make a permanent fix. Perhaps where the inversion continues without the missing site, but the empty data is included in the output to make post processing more consistent.