workbook parameterised constructors

Issue #298
- all 4 are simply duplicating existing behaviour, but perhaps we can get a more optimal version in future
- istream ctor is intended as an extension point that can then be used to create free/static functions to work with any future data source (vector), while the path ctor is a convenience function for the common case (from file)

include/xlnt/workbook/workbook.hpp

@@ -130,6 +130,31 @@ public:
     /// </summary>
     workbook();
 
+    /// <summary>
+    /// load the xlsx file at path
+    /// </summary>
+    workbook(const xlnt::path &file);
+
+    /// <summary>
+    /// load the encrpyted xlsx file at path
+    /// </summary>
+    workbook(const xlnt::path &file, const std::string& password);
+
+    /// <summary>
+    /// construct the workbook from any data stream where the data is the binary form of a workbook
+    /// </summary>
+    workbook(std::istream & data);
+
+    /// <summary>
+    /// construct the workbook from any data stream where the data is the binary form of an encrypted workbook
+    /// </summary>
+    workbook(std::istream &data, const std::string& password);
+
+    /// <summary>
+    /// Move constructor. Constructs a workbook from existing workbook, other.
+    /// </summary>
+    workbook(workbook &&other);
+
     /// <summary>
     /// Move constructor. Constructs a workbook from existing workbook, other.
     /// </summary>

source/workbook/workbook.cpp

@@ -500,6 +500,30 @@ workbook::workbook()
     swap(wb_template);
 }
 
+workbook::workbook(const xlnt::path &file)
+{
+    *this = empty();
+    load(file);
+}
+
+workbook::workbook(const xlnt::path &file, const std::string &password)
+{
+    *this = empty();
+    load(file, password);
+}
+
+workbook::workbook(std::istream &data)
+{
+    *this = empty();
+    load(data);
+}
+
+workbook::workbook(std::istream &data, const std::string &password)
+{
+    *this = empty();
+    load(data, password);
+}
+
 workbook::workbook(detail::workbook_impl *impl)
     : d_(impl)
 {
@@ -1178,9 +1202,7 @@ workbook::workbook(const workbook &other)
     d_->stylesheet_.get().parent = this;
 }
 
-workbook::~workbook()
+workbook::~workbook() = default;
-{
-}
 
 bool workbook::has_theme() const
 {