Pyp-beagle

- Fixing some Python 3 conversion issues
- Corrected a wrong import
- Enabling reading the ID column name of a photometric catalogue from the filters configuration file (it can still be overridden through the command line argument)

- Trying to solve CI error: can't find Rust compiler

- RuntimeError: latex was not able to process the following string:b'lp' solved following https://stackoverflow.com/a/32020370, i.e. redefining the minus sign in the preamble
- Recognizing use of env variables in the filters definition; skipping the header line in the ASCII filter transmission files
- Trying to solve a LaTeX error RuntimeError: latex was not able to process the following string: b'lp' using hints from here https://github.com/garrettj403/SciencePlots/issues/2 and here https://stackoverflow.com/a/53080504
- Adding LaTex use through our matplotlib configuration
- Updated env variables
- Updating README
- Adding missing import; adding configparser option since the default option has changed

- Python 3.8 not yet available on Shippable, using 3.7 instead
- Updating README to reflect move to Python 3

- Merge branch 'python_3' into develop
- Moving to support of Python 3 (3.8) exclusively
- Updated setup for Python 3
- Updated modules for Python 3

- Fix pypi failure: specifying that the README is in Markdown format, otherwise pushing to pypi fails, see https://pypi.org/help/#description-content-type and https://github.com/di/markdown-description-example
- Bug fix: the marginal photometry plot was failing when the filter configuration file contained bands not present in the observations, but only computed for the models
- Added option to specify flux units for photometry
- Update README.md