Add function and test unit for function to read BAM file

[lpantano]

Introduce a test unit to read the example GTF file at here: https://github.com/miRTop/mirtop/blob/dev/data/aligments/let7-perfect.bam

The output should be a nested dict like the following:

• top level key: each line in the BAM file.

Initial function already implemented here:
https://github.com/miRTop/mirtop/blob/dev/mirtop/bam/bam.py#L34

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

extra: this line should be optional: https://github.com/miRTop/mirtop/blob/dev/mirtop/bam/bam.py#L65. Need a parameter in the command line to turn it on and off.