Updates to sample import

[irzamsarfraz]

• Add ui and backend logic to import samples from multiple pipelines using file selectors instead of directories
• Fixed issue #688

[codecov[bot]]

Codecov Report

Patch coverage: 5.94% and project coverage change: -0.39 :warning:

Comparison is base (65dd1a3) 53.18% compared to head (d321f67) 52.80%.

:exclamation: Current head d321f67 differs from pull request most recent head 72bef02. Consider uploading reports for the commit 72bef02 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #709 +/- ## ========================================== - Coverage 53.18% 52.80% -0.39% ========================================== Files 79 79 Lines 12579 12679 +100 ========================================== + Hits 6690 6695 +5 - Misses 5889 5984 +95 ``` | [Impacted Files](https://app.codecov.io/gh/compbiomed/singleCellTK/pull/709?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compbiomed) | Coverage Δ | | |---|---|---| | [R/importMultipleSources.R](https://app.codecov.io/gh/compbiomed/singleCellTK/pull/709?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compbiomed#diff-Ui9pbXBvcnRNdWx0aXBsZVNvdXJjZXMuUg==) | `34.11% <5.94%> (-25.54%)` | :arrow_down: |

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