Pyimpspec

- Updated the .mpt parser to handle files without a metadata section.

- Fixed the BHT method (DRT analysis) so that it works properly when `num_procs` is set to greater than one and NumPy is using OpenBLAS.

- Added support for calculating the distribution of relaxation times using the `m(RQ)fit` method.
- Added `HavriliakNegamiAlternative` as an element with an alternative form of Havriliak-Negami relaxation.
- Added `ModifiedInductor` as an element for modeling non-ideal inductance.
- Updated the assertion message related to valid methods in the `calculate_drt` function.
- Updated the default lower and/or upper limits of some elements.
- Updated how `numpy.nan` is handled when creating `FittedParameter` objects.
- Refactored a minor portion of the TR-NNLS method (DRT analysis) code.
- Refactored the `pyimpspec.circuit.get_elements` function to ensure all circuit elements have unique symbols.
- Fixed a bug that caused the `Circuit.get_connections` method to return an empty list in some cases when invoked with `flattened=True`.
- Fixed bugs that caused the `Circuit.to_diagram` method to produce incorrect results.

- Added support for `kvxopt` as an optional dependency as a drop-in replacement for `cvxopt`.
- Updated how import errors related to the convex optimizers required by the TR-RBF method (DRT analysis) are handled, which should allow the rest of pyimpspec to function even if no convex optimizers can be imported successfully.

- Added the 3-sigma CI series to the legends of DRT plots.
- Updated the order that the mean and 3-sigma CI series are plotted in DRT plots.

- Updated API documentation.