Updated get_filtermatrix

[anthony-meza]

Updated get_filtermatrix, a function that generates a filter matrix. get_filtermatrix now has an optional argument ival that allows the user to decide which time step to start applying the filter.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.33% :tada:

Comparison is base (c0791ba) 28.31% compared to head (0f3a836) 32.64%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #65 +/- ## ========================================== + Coverage 28.31% 32.64% +4.33% ========================================== Files 4 4 Lines 1148 1121 -27 ========================================== + Hits 325 366 +41 + Misses 823 755 -68 ``` | [Files Changed](https://app.codecov.io/gh/ggebbie/ECCOtour.jl/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=G+Jake+Gebbie) | Coverage Δ | | |---|---|---| | [src/ECCOtour.jl](https://app.codecov.io/gh/ggebbie/ECCOtour.jl/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=G+Jake+Gebbie#diff-c3JjL0VDQ090b3VyLmps) | `34.83% <ø> (ø)` | | | [src/MatrixFilter.jl](https://app.codecov.io/gh/ggebbie/ECCOtour.jl/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=G+Jake+Gebbie#diff-c3JjL01hdHJpeEZpbHRlci5qbA==) | `22.93% <100.00%> (+22.93%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/ggebbie/ECCOtour.jl/pull/65/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=G+Jake+Gebbie)

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[anthony-meza]

cons_offset! is used in a script within ECCOonPoseidon, might be better suited there