Efficient FFI loading

[jorgemarpa]

This PR updates the FFI loading methods to be more memory efficient plus other minor changes.

Changes:

• loading FFIs using fitsio and reading only the asked data
• loading FFIs will attempt to preserve cadence quality. Useful when working with TESS FFIs.
• ffi.load_from_file() and tpf.from_TPFS() both accept the keyword sources for custom user input.
• ffi.flux_2d is a property useful for plotting functions
• saturated and bright pixels are masked into machine.source_mask and machine.uncontaminated_pixel_mask
• updated keyword arguments:
  • dist_lim --> sparse_dist_lim as machine attribute so it can be modified depending on instrument pixel scale (e.g. TESS)
  • time_corrector was deprecated and Perturbation handles this
  • machine.sources_flux_column to designate globally which column from the source catalog will be used as flux estimate. This update is necessary for TESS which uses the R band instead of G.
• Gaia query now also saves RP and BP flux columns
• plot_shape_model() has flexible radii and color mapping stretch from flux percentiles. This is useful for other instruments (TESS)

[jorgemarpa]

Need to run demo notebooks to check consistent results

[jorgemarpa]

lgtm

[jorgemarpa]

flake8 passing

[jorgemarpa]

flake8 passing