Generalisation of the Lightcurve Class

[transientlunatic]

The Lightcurve class, which is used to handle observational data within the package is currently suitable only for working with Kepler data.

An improved approach should include means of working with data from solar observatories such as GOES and RHESSI, with the option to work with other data sources, including neutron observations.

I propose to rewrite the Lightcurve class, while preserving backwards-compatibility, and utilising pydata/pandas, further, I propose incorporating `transientlunatic/lightcurve' as a Git submodule to do this, to allow the code to be reused outside the BayesFlare project.

An important consideration will be maintaining backwards-compatibility, and to do this a number of methods may need to be added purely for compatibility.