add function to read miRNA GTF file and get a dict as an output

[lpantano]

Introduce a test unit to read the example GTF file at here: https://github.com/miRTop/mirtop/blob/dev/data/examples/annotate/hsa.gff3

The output should be a nested dict like the following:

• top level key: hsa-let-7a-1
  • child key: hsa-let-7a-5p and hsa-let-7a-3p having as value the relative position to the top level, in this case, [94175957 - 94175962, 94176036 - 94175983] and [94175957 - 94176013, 94176036 - 94176033].

The two element list refers to the start and the end of the miRNA on the precursor.

Be aware that for negative strands the relative position are calculated different. end is start and start is end.

The unit test should be included here: https://github.com/miRTop/mirtop/blob/dev/test/test_functions.py#L22

[lpantano]

This is done for mirbase, but it would be good to support mirgeneDB at some point soon.