2.0.0 release

[grinner]

• Migrated primer3 header file Cython extern imports to thermoanalysis.pxi
• Optional C structure string argument c_ascii_structure added to _ThermoAnalysis methods to enable 3rd party use for structures
• Version bump to 2.0.0 due to breaking change
• Fix issue whereby no_structure is not correctly set to 1 when no secondary structure in found
• add list version of PRIMER_{PAIR, LEFT, RIGHT, INTERNAL} to design output dictionary keys. Retaining original keys as well for compatibility.
• Fix for missing thal_result.sec_struct and dpal_results.sec_struct initialization to NULL in recalc_primer_sec_struct

[mbiokyle29]

I stopped paying attention for like 6 months and yall are on 2.0! Congrats @benpruitt and @grinner and thanks for all the hard work.