Remove "abundances" from TransitDepthCalculator

[ideasrule]

Right now the transit depth calculator takes "abundances", which has to be a dictionary of 13 x 30 arrays spanning pressures from 10^-9 bar to 1000 bar, and temperatures from 100 to 3000 K. However, sometimes we don't have abundance information for this whole range because ggchem is unstable at very low temperatures. Is there a way to make the format of "abundances" more flexible? Can the user just specify abundances at points along the atmospheric profile? Can abundances be replaced with an AbundanceGetter object?

[ideasrule]

TransitDepthCalculator should support:

• reading from file
• specifying metallicity/CO
• perhaps specifying custom array

[ideasrule]

By default compute_depths now takes logZ and CO. The user can also specify custom_abundances, which can either be a file or a dictionary.