Vireadb

* Added number of threads to Minimap2 command (via `-t <THREADS>`) when adding FASTQ file(s) to a ViReaDB
* Previously, I only passed the number of threads to samtools (via `- <THREADS>`), but not Minimap2 (it used the default number of threads)

* Using lossy names in CRAM conversion is now optional (but is still used by default)
* Added optional verbosity to the `add_entry` function

* Added `numpy` to `install_requires`
* This causes `pip` to automatically install `numpy` when the user does `pip install vireadb`

* Made `minimap2` and `samtools` truly optional
* If `minimap2` isn't in `PATH`, the reference genome isn't indexed (`NULL` is stored in the database instead)
* If the user opens a ViReaDB database that was created without `minimap2`, but the user now has `minimap2` installed, the reference genome should automatically get indexed and added to the database

* Added `install_requires` with `pysam`
* This causes `pip` to automatically install `pysam` when the user does `pip install vireadb`

* Fixed bug handling insertions
* Exporting a FASTA will by default export all IDs if the user doesn't provide any