Adding a requirements.txt file to ease development

[linsalrob]

This PR adds a requirements.txt file that contains the required modules.

Steps to use development phanotate:

1. Fork the git repository to your github account
2. Clone the forked repository to your working machine (git clone git@github.com:username/PHANOTATE.git)
3. Create a virtual environment and activate it

   cd PHANOTATE
   virtualenv venv
   source venv/bin/activate

4. Check the location of your python or pip: which python or which pip
5. Install the dependencies: pip install -r requirements.txt
6. Make sure you have tRNAScan-SE in your conda environment (mamba install -c bioconda trnascan-se)