Easier-to-install version with optional dependencies

This release introduces two optional dependencies: celerite2 and h5py. These had previously been installation requirements, which caused problems for some users due to the complexity of installing those packages. We justified this change since celerite and h5py are not required for most routine use of muler. To install just the basic dependencies you can keep using: pip install muler. To install the extras you can now type:

pip install muler[extra]

We recommend folks attempt to install the extras, in case you find yourself needing those functionalities later.