Support gre protocol

[codecov-commenter]

Codecov Report

Patch coverage: 87.50% and project coverage change: +0.30% :tada:

Comparison is base (11dcd7c) 41.35% compared to head (e92f891) 41.65%. Report is 107 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #144 +/- ## ========================================== + Coverage 41.35% 41.65% +0.30% ========================================== Files 82 88 +6 Lines 8186 8493 +307 Branches 1269 1288 +19 ========================================== + Hits 3385 3538 +153 - Misses 4041 4190 +149 - Partials 760 765 +5 ``` | Flag | Coverage Δ | | |---|---|---| | tests | `41.65% <87.50%> (+0.30%)` | :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=CESNET#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [input/parser.cpp](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#diff-aW5wdXQvcGFyc2VyLmNwcA==) | `35.32% <87.50%> (-2.24%)` | :arrow_down: | ... and [78 files with indirect coverage changes](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/144/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET)

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[SiskaPavel]

Zmensil bych pocet commitu na 2:

1. Parsovani GRE protokolu
2. Integrace do parseru