Ipfix - add option to configure template export rate

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (0f52d43) 41.31% compared to head (44d9d74) 41.29%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #189 +/- ## ========================================== - Coverage 41.31% 41.29% -0.03% ========================================== Files 94 94 Lines 8660 8665 +5 Branches 1299 1299 ========================================== Hits 3578 3578 - Misses 4314 4319 +5 Partials 768 768 ``` | [Flag](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/189/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/189/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | `41.29% <0.00%> (-0.03%)` | :arrow_down: | 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=CESNET#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/189?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [output/ipfix.cpp](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/189?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#diff-b3V0cHV0L2lwZml4LmNwcA==) | `9.11% <0.00%> (-0.03%)` | :arrow_down: | | [output/ipfix.hpp](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/189?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#diff-b3V0cHV0L2lwZml4LmhwcA==) | `0.00% <0.00%> (ø)` | |

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