Get neighboring indices

[nealkruis]

1. Add public function to get the flattened index values for the grid points nearest to the target. This allows the user of Btwxt to query the values of external un-interpolated data with the same dimensions as the grid point data sets. Primary application of this is to determine the operation state of any neighboring points in ASHRAE Standard 205 data.
2. Fix up a few minor issues related to the Courier::Sender base class (see first commit).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.42%. Comparing base (1a8c0df) to head (3769900).

Files	Patch %	Lines
src/regular-grid-interpolator-implementation.cpp	95.24%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #41 +/- ## ========================================== + Coverage 88.42% 89.42% +1.00% ========================================== Files 6 6 Lines 501 520 +19 ========================================== + Hits 443 465 +22 + Misses 58 55 -3 ``` | [Flag](https://app.codecov.io/gh/bigladder/btwxt/pull/41/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bigladder) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/bigladder/btwxt/pull/41/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bigladder) | `89.42% <96.00%> (+1.00%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bigladder#carryforward-flags-in-the-pull-request-comment) to find out more.

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